Theresa May is set for the bleakest week of her time in power after leadership rivals publicly positioned themselves to grab the Tory crown if her Brexit plans collapse.

Ex-cabinet ministers Boris Johnson, Dominic Raab and Esther McVey all signalled a willingness to bid for the leadership amid speculation that Ms May faces a heavy defeat in the crunch Commons vote on her proposed Brexit deal.

More resignations were expected from the front bench in the run-up to the vote, with government insiders indicating it could still be delayed.

If she survives the first half of the week, Ms May is expected to head to Brussels where she will implore the EU to offer a concession on the hated “Irish backstop” so that she can try to sell the deal to Tory rebels one last time.

The prime minister spoke to president of the European Council Donald Tusk on Sunday, who said afterwards that it would be “an important week for the fate of Brexit”.

In London thousands of protestors waving union jacks joined a “Brexit betrayal” march sponsored by Ukip and addressed by far-right activist Tommy Robinson, while even more were said to have turned up to an anti-fascist counter-march.

The febrile atmosphere as the week starts is only set to intensify as MPs return to Westminster on Monday, with talk of Conservative plots and leadership challenges filling the air.

One Tory backbencher told The Independent: “No one knows if the prime minister is still going to be in Downing Street at the end of the week.

Tommy Robinson and Ukip lead Brexit 'betrayal' London protest amid huge police operation

“If she delays the vote, they might try and bring her down. If she loses heavily, they might try and bring her down. She is almost out of options.”

Former cabinet ministers from the Brexiteer wing of the party did little to dampen speculation of a potential contest as one by one they indicated a willingness to be party leader.

Boris Johnson, who has long coveted the top job, failed to exclude the possibility of standing against the prime minister in a potential contest during an interview on Sunday.

He said people do not want to hear about “leadership elections and personalities” right now, but said he would take “personal responsibility” for jobs lost if Britain pursued his Brexit plan instead of Ms May’s.

Sporting a well-groomed haircut that contrasted with his trademark ruffled look, he dismissed as “nonsense” reports that he had already offered ministerial jobs to MP colleagues in return for support.

Asked if he would rule out standing against Ms May, he responded: “I will give you an absolute categorical promise that I will continue to advocate what I think is the most sensible plan.

“I’m going to offer you the most sensible plan to get out of this mess.”

Earlier in the day ex-pensions secretary Ms McVey said she would support the PM if she went back to Brussels and renegotiated the deal, in particular the Irish backstop and the £39bn “divorce bill” that comprise part of the current withdrawal agreement.

Ex-cabinet minister Esther McVey says she would run for Conservative leader if asked

Asked if she would rule out running herself, she went on: “If people asked me then of course I should say, of course I should give it serious concern and do it – if people asked me.

“But at the moment I’m looking at who is in the papers, who can we get behind, but it shouldn’t be about the personality, it should be about the country.”

In an interview broadcast shortly after Ms McVey’s, Mr Raab was also asked if he would rule himself out for a run at the top job.

The former Brexit secretary said: “I’ve always said I wouldn’t rule it out, but I’m just not going to get sucked into that debate.

“The public and people watching this show would think it would be very self-indulgent to be engaging in that speculation.”

He would not comment when asked if he would stand aside for fellow Brexiteer Mr Johnson to run for the leadership.

Dominic Raab says he would not rule out running for Conservative leader

Earlier in the weekend, May ally Amber Rudd appeared to pre-empt the result of Tuesday’s vote on the PM’s plans by saying she would back a Norway-style settlement for the UK.

The pensions secretary, from the more liberal wing of the party, drew attention to her own leadership credentials by launching a searing attack on the Brexiteer wing of the party, whose champions she said “flounce out a lot”.

The intense strain that Brexit is putting on the wider Conservative party also began to show with one minister accepting some MPs may quit in the days ahead.

“I think one or two people might leave, I don’t know who they will be,” Brexit minister Kwasi Kwarteng told BBC Radio 5 Live’s Pienaar’s Politics.

“They might decide to retire or they might decide to abandon the party whip, that has happened before.”

Former attorney general Dominic Grieve also suggested that his party could split over Brexit.

Brexit casualties Show all 10 1 /10 Brexit casualties Brexit casualties Andrea Jenkyns - Resigned from Parliamentary Private Secretary at the ministry for housing, communities and local government role May 2018 - The Morley and Outwood MP said: “We want to see a new relationship with Europe, with a new model not enjoyed by other countries – nothing that leaves us half-in, half-out. “And in order to achieve this, we need to leave the customs union.” Ms Jenkyn’s also said she wished to dedicate more of her time to Parliament’s influential Exiting the European Union select committee, after a series of “unbalanced” reports produced by MPs PA Brexit casualties David Davis - Resigned from Secretary of State for Exiting the European Union role July 2018 - quit following a major row with May over her plans for post-Brexit relations with the EU. Davis resignation letter said: “As you know there have been a significant number of occasions in the last year or so on which I have disagreed with the Number 10 policy line, ranging from accepting the [European] Commission’s sequencing of negotiations, through to the language on Northern Ireland in the December Joint Report. “At each stage I have accepted collective responsibility because it is part of my task to find workable compromises, and because I considered it was still possible to deliver on the mandate of the referendum, and on our manifesto commitment to leave the Customs Union and the Single Market. “I am afraid that I think the current trend of policy and tactics is making that look less and less likely.” He went on to argue that the “general direction” of Ms May’s policies would leave the UK “in at best a weak negotiating position, and possibly an inescapable one”. AFP/Getty Brexit casualties Steve Baker - Resigned from Minister at the Department for Exiting the European Union role July 2018 - Mr Baker, a key Tory figure in the Leave campaign, was David Davis’s main lieutenant at Dexeu, and was hailed as ”courageous and principled” by other Brexiteer Tories as he also left. Reuters Brexit casualties Boris Johnson - Resigned from Foreign Secretary role July 2018 - resigned over May's Chequers plan. In his resignation letter to the prime minister, Mr Johnson said: "On Friday, I acknowledged that my side of the argument were too few to prevail and congratulated you on at least reaching a Cabinet decision on the way forward. "As I said then, the government now has a song to sing. "The trouble is that I have practised the words over the weekend and find that they stick in the throat." Reuters Brexit casualties Conor Burns - Resigned from Parliamentary Private Secretary to Foreign Secretary role July 2018 - A Brexit supporter who worked alongside Boris Johnson stated in his resignation letter: “I've decided it's time to have greater freedom. I want to see the referendum result respected. And there are other areas of policy I want to speak more openly on.” Rex Brexit casualties Chris Green - Resigned from Department for Transport role July 2018 - The Bolton West MP said: "Parliament overwhelmingly decided to give the decision of whether to leave or remain in the European Union to the British people and they made an unambiguous decision that we ought to leave. "I have always understood the idea in 'Brexit means Brexit' is that the final deal should be clear to me and my constituents - that we have, in no uncertain terms, left the European Union. Twitter Ads info and privacy "The direction the negotiations had been taking have suggested that we would not really leave the EU and the conclusion and statements following the Chequers summit confirmed my fears. "I recognise that delivering Brexit is challenging, however I had hoped at tonight's meeting that there would be some certainty that my fears were unfounded but, instead, they have been confirmed. "I have been grateful for the opportunity to serve as Parliamentary Private Secretary and it is with regret that I offer my resignation with immediate effect." PA Brexit casualties Maria Caulfield - Resigned from Conservative Party vice-chair for women role July 2018 - resigned over May's Chequers plan. Lewes MP warned that the direction of travel did “not fully embrace the opportunities that Brexit can provide”. Ms Caulfield said in her letter to the PM: “The policy may assuage vested interests, but the voters will find out and their representatives will be found out. This policy will be bad for our country and bad for the party. “The direct consequences of that will be prime minister Corbyn.” PA Brexit casualties Ben Bradley - Resigned from Conservative Party vice-chair for young people role July 2018 - resigned over May's Chequers plan. The Mansfield MP said: “I admit that I voted to Remain in that ballot. What has swayed me over the last two years to fully back the Brexit vision is the immense opportunities that are available from global trade, and for the ability for Britain to be an outward looking nation in control of our own destiny once again. “I fear that this agreement at Chequers damages those opportunities; that being tied to EU regulations, and the EU tying our hands when seeking to make new trade agreements, will be the worst of all worlds if we do not deliver Brexit in spirit as well as in name, then we are handing Jeremy Corbyn the keys to No10.” PA Brexit casualties Robert Courts - Resigned from Parliamentary Private Secretary role July 2018 - resigned over May's Chequers plan. MP Mr Courts said: “I have taken a very difficult decision to resign my position as [parliamentary private secretary] to express discontent with the Chequers [plans] in votes tomorrow. “I had to think who I wanted to see in the mirror for the rest of my life. I cannot tell the people of Woxon that I support the proposals in their current form.” Getty Brexit casualties Scott Mann - Resigned from Parliamentary Private Secretary role July 2018 - resigned over May's Chequers plan. "I fear elements of the Brexit white paper will inevitably put me in direct conflict with the views expressed by a large section of my constituents. I am not prepared to compromise their wishes to deliver a watered-down Brexit. "The residents of North Cornwall made it very clear that they wish to have control over our fishery, our agricultural policy, our money, our laws and our borders. I will evaluate those principles against the Brexit white paper and ensure that I vote in line with their wishes." Rex

Tory MP Will Quince quit as a ministerial aide to defence secretary Gavin Williamson, while the Sunday Telegraph reported another aide was on the verge and cabinet Brexiteer Penny Mordaunt was also considering her position.

Responding to speculation that Ms May could delay Tuesday’s vote to secure fresh concessions from Brussels or win over wavering MPs, Brexit secretary Stephen Barclay insisted it would go ahead as planned.

But another cabinet source told The Independent the picture was less clear, and that the silence from Downing Street on the issue was “deafening”.

Ms May’s call with Mr Tusk only became public when the European politician tweeted about it, saying: “I had a phone call with PM [Theresa May]. It will be an important week for the fate of Brexit.”

Labour is considering its options if Ms May is defeated and would hold talks with other opposition parties about how to proceed.