THERESA May is “a dead woman walking” who will inevitably face a challenge for the Tory Party leadership, George Osborne has said, as he suggested the Prime Minister should be forced to resign imminently.

The former chancellor, who was sacked by May when she became PM after the EU referendum, said: “We could easily get to the middle of next week and it all collapses for her. I think we will know very shortly.”

Osborne blamed May’s campaign for a General Election result he said had undone the work of himself and former prime minister David Cameron in winning seats such as Bath, Brighton Kemptown and Oxford East, now lost to Labour and the LibDems.

“She is a dead woman walking and the only question is how long she remains on death row,” the editor of the Evening Standard said.

Osborne spoke out as it was reported Boris Johnson was poised to put his name forward for the leadership. His chances were scuppered last year when his former ally Michael Gove went back on his word and ran for the leadership himself. Osborne said blame for the Tories’ loss of their House of Commons majority should be on the shoulders of May, though her advisers Nick Timothy and Fiona Hill resigned on Saturday.

“You can’t just blame the advisers,” he said. “The only person who decides to have an election is the Prime Minister, the person who decides what’s in the manifesto is the Prime Minister.”

But Defence Secretary Sir Michael Fallon said May was changing her leadership style. He added: “There is no other party that has any legitimacy or credibility.”

After losing her majority on Thursday in a disastrous result for the Tories, May has been forced to try and do a deal with Northern Ireland’s Democratic Unionist Party (DUP) to stay in Number 10 Downing Street.

Yesterday, she battled on, attempting to bolster her position, making one of her closest Cabinet allies effectively Deputy Prime Minister. Damian Green, who was the work and pensions secretary, was appointed First Secretary of State – a title held by William Hague under David Cameron – as well as Minister for the Cabinet Office.

In other moves David Gauke, who was the Treasury chief secretary, was promoted to take over at the Department for Work and Pensions, while David Lidington, the leader of the Commons, also got a step up as the new Lord Chancellor and Justice Secretary.

May had already announced that her five most senior ministers – including Chancellor Philip Hammond and Foreign Secretary Johnson – were carrying on in their current positions.

Meanwhile, Jeremy Corbyn vowed to lead Britain out of the EU as he insisted Labour was “ready any time” for another General Election.

The party leader said Labour would “absolutely” ensure Brexit occurs if they secured power, with a focus on negotiating tariff-free access as part of a “jobs-first Brexit”.

Labour won 262 seats in the General Election, up from the 232 secured by Ed Miliband in 2015, but the Conservatives remain the largest party in Parliament.

Speaking on the BBC’s Andrew Marr Show, Corbyn said: “We’re going to put down a substantial amendment to the Queen’s Speech which will contain within it the main points of our manifesto, and so we’ll invite the House to consider all the issues we put forward which I’ve mentioned – jobs-first Brexit, issues of young people and austerity, there are many.”

On Brexit, Corbyn said he wants his version of Brexit negotiated as quickly as possible along with the guaranteeing of the post-Brexit rights of EU nationals living in the UK.

Asked if Labour would seek to keep Britain in the European single market and customs union, Corbyn said: “Well, the single market is a requirement of EU membership and since we won’t be EU members there will have to be an arrangement made.”

Pressed on whether he was being clear that the UK will leave the EU, Corbyn said: “Absolutely ... we want tariff-free access to the European market, we also want to maintain a very important university and research collaboration in Europe, and there’s a whole host of European agencies – Euratom, security, environment – we wish to be part of.

“We will absolutely remain part of the European Convention on Human Rights and European Court of Human Rights – we are not walking away from those vital post-war agreements that were made.”

On the so-called Great Repeal Bill, designed to transfer EU law into British law to enable changes, Corbyn said: “The Great Repeal Bill, I suspect, has now become history.”

Asked if there will be another General Election this year, Corbyn said: “I think it’s quite possible there’ll be an election later this year or early next year, and that might be a good thing because we cannot go on with a period of great instability.”

Questioned on whether he was in it for the long-term, Corbyn said: “Look at me, I’ve got youth on my side.”

Today Arlene Foster, leader of the DUP, will today hold talks with May in Downing Street over an agreement to prop up the Tory minority government. May has faced considerable criticism over the pact.