Ben Wallace is currently the bookies’ favourite to succeed Boris Johnson as Tory leader and prime minister, having risen up the party’s popularity rankings in recent months.
The latest YouGov poll of Conservative Party members shows the defence secretary as the clear favourite for next leader.
Mr Wallace beats all the main contenders including Penny Mordaunt, Rishi Sunak and Foreign Secretary Liz Truss.
But who else is likely to feature in the leadership contest?
Who has already said they are running?
Former chancellor Rishi Sunak is the biggest name to have entered the contest so far.
Up until the last few months, he was seen by many Conservative MPs as the frontrunner in the race to succeed Mr Johnson.
Rishi Sunak launches bid to become next prime minister following Boris Johnson’s resignation
Boris Johnson resigns: Tory grandees want new prime minister in place by early September
Boris Johnson ‘a risk to the country’ over private meeting with ex-KGB agent Alexander Lebedev
An exclusive poll for Sky News in January found that almost half of Tory members thought Rishi Sunak would make a better leader and could win more seats at the next election than Mr Johnson.
But after introducing a number of policies – such as increasing National Insurance contributions – that went down badly with Tory MPs, his popularity has slumped.
Having also been issued with a fixed penalty notice for attending the PM’s birthday party during lockdown, his ability to separate himself from partygate has been compromised.
The revelation his wife, multi-millionaire Akhshata Murty, held non-dom status and therefore did not have to pay UK tax on her sizeable international income also damaged his standing.
Following an uproar that a minister’s wife was not paying UK taxes on that income, she confirmed she would do so as it had “become clear that many do not feel it is compatible with my husband’s role as chancellor”.
Mr Sunak had been in Number 11 since early 2020, being elevated straight from a junior ministerial post to one of the most powerful cabinet positions after Sajid Javid’s surprise resignation.
Within a matter of weeks COVID struck and the former chancellor assumed a prominent role in the government’s pandemic response, announcing a raft of measures to support workers and businesses.
His slick social media graphics promoting his various policies have been seen as an attempt to build his personal brand and pave the way for a future leadership bid.
Mr Sunak was elected in 2015, succeeding former Tory leader Lord Hague in the seat of Richmond in North Yorkshire.
He backed Brexit in 2016, telling his constituents at the time that it was the “toughest decision” of his political career.
Tom Tugendhat, the current chair of the Foreign Affairs Select Committee, became the first to throw his hat in the ring as a Conservative leadership candidate after Mr Johnson left office.
Mr Tugendhat, a former soldier, was heavily critical of the government’s handling of the Afghanistan crisis and could be a possible figurehead for the so-called One Nation conservative wing, particularly if Jeremy Hunt chooses not to stand.
Announcing his future bid for leader with an article in the telegraph, Mr Tugendhat said: “I have served before – in the military, and now in parliament. Now I hope to answer the call once again as prime minister.
“It’s time for a clean start. It’s time for renewal.”
Although lacking in ministerial experience, Mr Tugendhat’s name is increasingly coming up as a viable option for the next Conservative Party leader.
The Foreign Affairs Select Committee chairman supported the campaign to remain in the EU but voted loyally on Brexit matters under both former PM Theresa May and Boris Johnson.
He has opposed stricter coronavirus measures during the pandemic.
Who else could stand to replace Mr Johnson?
The foreign secretary has long been touted as a potential successor to Mr Johnson, and has been shown to be popular among Conservative party members in surveys.
Ms Truss, like Mr Sunak, is widely seen to have been laying the groundwork for a future tilt at the top job with her social media output.
She has been an MP since 2010 and began rising up the ministerial ladder soon after entering parliament.
She is currently the longest continuously serving member of the cabinet, having held positions under David Cameron, Theresa May and Boris Johnson.
“Liz Truss: the new Iron Lady?” read the headline of a profile piece in The Times just before Christmas.
Any comparisons to Margaret Thatcher, Tory PM from 1979 to 1990 and a figure adored by the party faithful, will only help Ms Truss, who seems all too willing to play up any likeness.
Defence Secretary Ben Wallace has risen up the popularity rankings among party members in recent months and is said to be discussing a leadership bid with his wife and three children before taking a decision to run.
In a YouGov poll released before Mr Johnson announced he was quitting, Mr Wallace came out as a clear favourite to succeed the PM.
He has won praise from backbenchers in Westminster for his calm under pressure and handling of the Ukraine crisis.
His role in overseeing the evacuation of refugees and British nationals from Afghanistan was also applauded and he continues to be a key voice in the UK’s response to the Russian invasion of Ukraine.
Mr Wallace also has ties to Scotland, having both previously been an MSP and commissioned as a member of the Scots Guard before entering Westminster.
This could be favourable if the government faces continuing calls for a second Scottish independence referendum.
A long-term ally of Boris Johnson, he has claimed he has “no interest” in the leadership and feels privileged to be in the position he currently holds.
But if concern over changing leader during a time of war is playing on the minds of Tory MPs, could Mr Wallace be persuaded to step up as the candidate best placed to ease those nerves?
The current chair of the health and social care select committee was Boris Johnson’s rival in the head-to-head run-off in the last Conservative leadership election in 2019.
The dividing line in that contest was Brexit – and Mr Johnson’s vow to leave the EU even without a deal saw him win 66% of the vote.
After the contest Boris Johnson surprised many by sacking Jeremy Hunt as foreign secretary.
While Mr Hunt appears to have grown to enjoy his role as a party grandee and chair of a select committee, he has repeatedly refused to rule out another run at the leadership.
His backers believe his strength as a candidate would come from not being tainted by being part of Mr Johnson’s cabinet.
Sajid Javid is “seriously considering” running for Tory leader, sources say.
He has held almost every senior cabinet position – he was most recently health secretary, but has served as chancellor, home secretary, housing secretary, business secretary and culture secretary.
He previously stood for the leadership on a joint ticket with former cabinet minister Stephen Crabb in 2016, and as a candidate in his own right in the 2019 race, but failed to secure enough support to make the final run-off.
Numerous profiles have been written about his remarkable backstory – the son of a Pakistani immigrant bus driver, who lived above a shop in Bristol and became a high-flying investment banker and then politician.
He returned to Boris Johnson’s cabinet last summer having dramatically resigned as chancellor at the start of 2020 when he refused to allow Number 10 to choose his team of advisers.
During his time on the backbenches he made efforts to portray himself as a Thatcherite – her portrait hangs in his office — and was critical of some COVID measures, but his support for the Remain campaign in the Brexit referendum, and backing of Plan B COVID measures, may put off some Conservative MPs.
For some time Priti Patel was seen as the darling of the Conservative grassroots, with a proud right-wing stance on immigration and public spending.
She was a major backer of Boris Johnson’s leadership bid in 2019 and was rewarded by being appointed home secretary.
But since taking on that role her star appears to have faded, with her handling of the small boat crossings in the Channel being a source of significant criticism.
Her popularity among the right wing of the parliamentary party means her candidacy should not be written off, but questions over her handling of the Channel migrant crisis have likely dented her chances.
However, a report published in November 2020 by the prime minister’s then adviser on ministerial standards, Sir Alex Allan, had found the home secretary had breached the ministerial code with behaviour that amounted to bullying.
Mr Johnson overruled, saying the code hadn’t been broken and she could keep her job.
Ms Patel issued an “unreserved apology to anybody who has been upset by anything that has taken place”.
Names to keep an eye on
Nadhim Zahawi is another politician who some believe could make a case for the leadership. Having served in various junior ministerial posts, his success as vaccines minister saw him promoted to education secretary.
He has since replaced Rishi Sunak as chancellor.
Seen as a strong communicator and as someone relatively untarnished compared to some other cabinet ministers, many will be watching closely to see if Mr Zahawi puts his hat into the ring.
As a nine-year-old, Mr Zahawi and his Iraqi Kurd family fled from Saddam Hussein to the UK and he has told of how he was unable to read English at 11 years old.
In a deeply personal speech in October last year, Mr Zahawi spoke of his own experiences arriving in Britain as a child refugee and said the UK “took in a young Kurdish boy without a word of English and made him a Cabinet minister”
“Now it’s my turn to make sure that the opportunities that transformed my life are available to every child in every corner of our great country,” he said.
Another potential candidate is Penny Mordaunt, who was sacked as defence secretary when Boris Johnson came to power.
Having kept a relatively low profile since returning to government as an international trade minister, she is nonetheless seen as popular with Conservative MPs and ambitious.
Ms Mordaunt, who is seen by some as a possible dark horse of a future Conservative leadership contest, has said she was “shocked at the stupidity of what has taken place” in Downing Street with regards to the partygate row.
For Tory backbenchers who want to see a change in political direction of the government, particularly when it comes to the approach to the pandemic, Mark Harper could be a possible option.
The former chief whip was eliminated in the first round when he ran for the leadership in 2016, but his role as co-chair of the lockdown sceptic COVID Recovery Group has seen him act as a spokesman for a wing of the party over the last year.
Attorney General Suella Braverman said “it’s time to go” for Mr Johnson and said she would put her name into the ring if there is a leadership contest.
Ms Braverman said the prime minister had handled matters “appallingly” in recent days.
She said: “The balance has tipped now in favour of saying that the prime minister – it pains me to say it – but it’s time to go.”
Before the prime minister announced his resignation, Ms Braverman indicated that she would compete in a Tory leadership contest.
The current transport secretary is understood to have been one of the first senior Tory MPs to be seriously considering a leadership bid after Mr Johnson’s resignation.
He is one of the most recognisable senior Tories, appearing for the government in many a morning media round and appears to have a penchant for acting as his videos promoting various transport initiatives have shown.
The MP for Welwyn Hatfield since 2005, he went to a grammar school then Manchester Polytechnic and in 1989, aged 21, he was in a coma for a week after a car crash in Kansas, in the US.
He was a photocopier sales rep before owning a print business and founded a publishing business with his wife while he recovered from cancer.
Mr Shapps has ended up in hot water over the years for allegedly using pseudonyms and having a second job while he was an MP, which he denied then eventually admitted in 2015.
He was a favourite of David Cameron, who made him housing secretary and Conservative Party chairman.
But he stepped down from his post in 2015 over allegations of bullying within the Conservative Party – which he was not involved in – and had a few years on the back benches under Theresa May’s reign.
Mr Johnson rewarded him for supporting his leadership bid, despite being a Remainer, by making him transport secretary.
He has essentially nationalised Northern Trains and has been in charge of COVID travel restrictions. He is currently dealing with potential airline staff strikes and airport chaos.
