Boris Johnson refused to rule out challenging Theresa May for the Tory leadership as he warned her Brexit deal left the UK open to “blackmail” by Brussels.

The former foreign secretary said it was “nonsense” to suggest he had already begun offering jobs in a future Johnson administration to fellow Tories, but sidestepped the opportunity to promise not to stand against the Prime Minister.

He said her Brexit deal could get through the Commons if it was stripped of the backstop measure – an insurance policy to prevent a hard border with Ireland – insisting that would be “relatively simple” to achieve.

Another Brexiteer ex-Cabinet minister, Esther McVey, said she would give “serious concern” to standing for the leadership.

Mr Johnson, one of the leading players in the Leave campaign in the referendum, said people should not “underestimate the deep sense of  personal responsibility I feel for Brexit”.

Mrs May’s future hangs in the balance, with a heavy defeat in Tuesday’s Commons showdown likely to lead to fresh pressure on her leadership.

Asked to give an “absolute, categorical promise” that he would not stand against the Prime Minister, Mr Johnson said: “I will give you an absolute, categorical promise that I will continue to advocate what I think is the most sensible plan.”

Challenged on whether he had already begun speaking to colleagues to offer them roles in his future government, Mr Johnson told BBC’s Andrew Marr Show: “I can tell you that’s nonsense.”

Setting out his Brexit plan, Mr Johnson said resolving the Irish border issue should be postponed so it forms part of the talks on a future trade deal and the UK should withhold a “substantial chunk” of the £39 billion divorce bill until that deal is done.

Preparations should also be stepped up for a no-deal Brexit, he said.

He admitted he would feel personally responsible if people lost their jobs in a no-deal departure from the EU.

“Of course I will,” he said. “Do not underestimate the deep sense of personal responsibility I feel for Brexit and for everything that has happened.

“Do not underestimate how much I care about this because this is fundamental to our country and it absolutely breaks my heart to think that -after all that we fought for, all that we campaigned for, all that  (Brexit Secretary) Steve Barclay campaigned for, everybody believes in – we should consign ourselves to a future in which the EU effectively rules us in many respects and yet we have no say round the table in Brussels.

“That is an absurdity. We cannot go down that route.”

Asked about her leadership ambitions, Ms McVey told Sky News’ Sophy Ridge on Sunday: “I’ve seen the array of people who have come forward at the moment and I think if we can all get behind one – for me the most important thing is not the personalities, it is the deal for our country needs to be better.”

She added: “If people asked me, then of course you’d give it serious concern and do it if people asked me but at the moment I’m looking at who’s in the papers, who we can get behind but it shouldn’t be about the personality, it should be about the country and this deal.”

Former Brexit secretary Dominic Raab refused to rule out a leadership bid, relling Sky News he would not “get sucked into that debate”.