Theresa May is facing mounting pressure to resign after Commons leader Andrea Leadsom quit the cabinet over the prime minister’s Brexit plan.

Ms Leadsom resigned with a “heavy heart” on the eve of the European elections, saying she no longer believed the government’s approach would deliver the referendum result the people voted for.

The prime minister continued to cling on to power on Wednesday after Sir Graham Brady, the chairman of the 1922 Committee, failed to agree a rule change which would allow a fresh bid to topple her.

Following the failed bid to oust her in 2018, Ms May would be safe from another confidence motion until December under the existing rules.

She is expected to meet Sir Graham on Friday – the day after the EU elections, which are expected to be disastrous for the Conservatives.

The leadership race looks likely to be a crowded field, with former foreign secretary Boris Johnson the bookmakers’ favourite.

His closest rivals on the Eurosceptic wing of the party are former Brexit secretary Dominic Raab and environment secretary Michael Gove, who scuppered Mr Johnson’s leadership bid in 2016.

Senior ministers are refusing to confirm whether they will stand until after Ms May announces her departure date, but international development secretary Rory Stewart has confirmed his ambitions for the top job.

Foreign secretary Jeremy Hunt, home secretary Sajid Javid, health secretary Matt Hancock, treasury chief secretary Liz Truss and former Commons leader Andrea Leadsom are all thought to be considering a run at the leadership.

Who could succeed Theresa May as Conservative leader? Show all 9 1 /9 Who could succeed Theresa May as Conservative leader? Who could succeed Theresa May as Conservative leader? Boris Johnson Former foreign secretary Boris Johnson has long been hopeful, he previously stood in the leadership contest that followed the Brexit vote and has at many times since been thought to be maneuvering himself towards the goal. He remains a darling of the party's right wing, particularly those in the ERG, and is the most popular choice among Tory voters but his leadership bid would be fiercely opposed by many MPs PA Who could succeed Theresa May as Conservative leader? Michael Gove Environment secretary Michael Gove is another member who has long wanted to be leader. He has lately been known for rousing his party in the commons, his recent speeches on the Brexit deal and Labour's no confidence motion have overshadowed the Prime Minister's. He has been loyal to the Prime Minister, partly to shed his reputation as a backstabber who abandoned Boris Johnson to stand against him in the 2016 leadership election Getty Who could succeed Theresa May as Conservative leader? Dominic Raab Former Brexit secretary Dominic Raab has emerged as a favourite to be the Brexiteer candidate in a contest to succeed to Ms May. He displayed a grip on detail in his role as Brexit secretary. When asked recently if he would like to become prime minister he replied "never say never" Getty Who could succeed Theresa May as Conservative leader? Rory Stewart International development secretary Rory Stewart is pitching himself as the sensible candidate, promising to rule out both a second referendum and a no-deal Brexit. He was only recently promoted to the cabinet, previously serving as prisons minister, where he caught headlines with a pledge to resign if he could not reduce levels of violence within a year PA Who could succeed Theresa May as Conservative leader? Esther McVey The former work and pensions secretary announced that she will be standing for the leadership when May leaves. McVey is the first to explicitly state that she intends to stand. She resigned from the cabinet in protest over May's Brexit deal AFP/Getty Who could succeed Theresa May as Conservative leader? Sajid Javid Home secretary Sajid Javid is said to have a plan in place for a leadership race. He made headlines over Christmas when he declared that people smuggling over the English channel was a "major incident" and more recently when he revoked the citizenship of ISIS bride Shamima Begum. Son of a bus driver, he wants the Conservatives to be seen as the party of social mobility PA Who could succeed Theresa May as Conservative leader? Jeremy Hunt Foreign secretary Jeremy Hunt was recently thought to be the favourite in the event of a leadership race as he could sell himself as the man to unite the party. Critics worry that his long stint as health secretary could return to haunt him at a general election. He has reportedly been holding meetings with Tory MPs over breakfast to promote his leadership PA Who could succeed Theresa May as Conservative leader? Andrea Leadsom Following the Prime Minister's second defeat over her Brexit deal, Leader of the house Andrea Leadsom hosted a dinner party at which "leadership was the only topic of conversation", The Times heard. Leadsom ran against Theresa May in the 2016 leadership election before dropping out, allowing May to become Prime Minister AFP/Getty Who could succeed Theresa May as Conservative leader? Priti Patel Former international development secretary Priti Patel is thought to be positioning herself as a contender. One MP told The Independent "she knows she's from the right of the party, the part which is going to choose the next leader, so she's reminding everyone she's there." Patel left the government late in 2017 after it emerged that she had held undisclosed meetings with Israeli officials PA

Here are May’s most likely successors:

Johnson resigned from the cabinet in July (PA)

Boris Johnson, former foreign secretary

Resigned from the cabinet over May’s Chequers plan last July. Has argued that a no-deal exit would reflect the result of the 2016 referendum. The former mayor of London was once seen as the “Heineken candidate” capable of reaching voters other Tories could not. But his star has waned among Tory MPs left unimpressed by his two years as foreign secretary.

His popularity among Tory activists would give him a strong chance of winning if he made the final shortlist, but he might be blocked by a determined “stop Boris” campaign already being planned by his critics. If successful, the move would anger many Tory activists.

Leadsom’s leadership hopes have been revived (Getty) (Getty Images)

Andrea Leadsom, former Commons leader

Resigned from the cabinet on the eve of the European elections, saying she no longer believed the government’s approach would deliver the Brexit referendum result the people voted for.

She showed her inexperience in the 2016 leadership election, withdrawing to leave May as the last candidate standing after making comments appearing to suggest she would make a better leader because she had children.

The former Commons leader has strong support among Brexiteers.

Javid is one of the frontrunners to replace Theresa May (Jack Taylor/Getty)

Sajid Javid, home secretary

Backed Remain in 2016 referendum despite his Eurosceptic instincts. Now a keen Brexiteer, and prepared to keep a no-deal exit on the table. Has consistently challenged May in cabinet meetings.

Ambitious and has a brilliant backstory as the son of Pakistani bus driver who came to the UK with £1 in his pocket. But appears to be suffering from the curse of being one of the frontrunners in the Tory race. His stock has fallen recently after what some MPs see as unforced errors such as his handling of migrants crossing the English channel and his decision to strip Isis schoolgirl Shamima Begum of her UK citizenship.

Hunt would pitch himself as a unifier (PA)

Jeremy Hunt, foreign secretary

Remainer who briefly supported a Final Say referendum on the Brexit deal after the 2016 vote. Now rejects the idea, and is seen as a born-again Brexiteer who also wants to keep a no-deal departure on the table.

His pitch would be as the candidate who could unite the Tories’ warring factions. Tory members might judge him too much of a Remainer. Critics worry that his long stint as health secretary could return to haunt him at a general election. Could benefit from a slide in Javid’s fortunes, but will not want to overtake him to become the frontrunner.

Gove headed the Vote Leave campaign alongside Boris Johnson (PA)

Michael Gove, environment secretary

Along with Johnson, headed the victorious Vote Leave campaign in the 2016 referendum. Even his critics admit he is one of the few successful departmental ministers since his surprise return to May’s cabinet in 2017, a year after she sacked him.

Has been a loyal and articulate promoter of the withdrawal agreement. Critics see this as in part an attempt to shed his reputation as an assassin who turned on Johnson to run for the leadership in 2016.

Gove’s refusal to join other Brexiteers in resigning from the cabinet might count against him among Eurosceptics. However, many of them are now coming round to his strategy of getting the deal “over the line” to save Brexit.

Raab could be a dark horse in the contest (PA)

Dominic Raab, former Brexit secretary

Eurosceptic former chief of staff to David Davis, and followed his path by becoming and then resigning as Brexit secretary, complaining he was kept out of the loop by May and her civil service negotiator Olly Robbins. Displayed a grip on the detail while in the job, and impressed May allies in the 2016 referendum when he campaigned for Leave. But did not impress Brussels officials.

A fresh face, although his relative lack of cabinet experience could count against him. But a dark horse to watch in a leadership race. Has a big decision to make: will he back May’s deal if Johnson does, or decide to have clean hands and oppose it to the end, and potentially be on the losing side?

Hancock is a survivor from the Cameron regime (PA)

Matt Hancock, health secretary

A new entrant in the leadership race and, at 40, could be the youngest candidate. A rare breed in the May government as a survivor of David Cameron’s regime: before becoming an MP in 2010, he was chief of staff to George Osborne. Impressed as culture secretary, as one of the few UK politicians to understand the digital era and power of the internet giants. Has also shown grip in a testing job since his promotion to health and social care secretary.