Tory Brexiteers are bullying Theresa May and behaving intolerably, Sir John Major has said.

The former prime minister famously branded rebellious Eurosceptics during his premiership “bastards”, but said those making life difficult for Mrs May were even worse.

It comes as former prime minister Tony Blair also made a fresh Brexit intervention, with a plea to Labour MPs to “hold firm” against withdrawal.

Mrs May is meeting a small group of senior ministers to update them on progress in the negotiations and Downing Street said there were “big issues still to resolve” in talks with the EU.

In Brussels, European Commission officials said there had been “no breakthrough yet”.

Cabinet minister Esther McVey, not one of those attending the meeting with Mrs May, pointedly refused to endorse the Chequers plan, although she insisted the Prime Minister had her full support.

Invited by the BBC to offer her backing to Mrs May’s Chequers blueprint, the Work and Pensions Secretary said: “I am completely supportive of the Prime Minister.”

She added: “I am fully 100% behind the Prime Minister and we will get the best deal for this country.”

The Democratic Unionist Party, which props up the minority government, has piled on the pressure by warning it could vote against the Budget if the PM breaks their red lines.

But Mrs May is also battling intense internal Tory divisions as she prepares for next week’s crunch summit in Brussels.

Sir John Major criticised Conservative MPs who threaten to launch a no-confidence bid in the PM if she refuses to bow to their demands.

He told the BBC’s Political Thinking podcast: “I have great sympathy for her plight and I think the way she is being treated by some of her colleagues is absolutely outrageous.”

Sir John attacked the “bullying” of the PM by some in the party.

Asked about the “bastards” from his era, he replied: “Their behaviour was pretty intolerable, but not nearly as intolerable as the way the present Prime Minister is being treated.”

In an indication of the febrile atmosphere within the Tory ranks, an MP suggested that the process for replacing Mrs May could be completed in a matter of days.

Mark Pritchard said: “Lots of wild and loose talk about leadership moves. There is no vacancy.

“However, on a technical point, if a vacancy did arise process need not take more than two working weeks – four days in Commons (if needed) and six days with membership – does not need to be an overly long process.”

Mrs May appealed on Wednesday for MPs to come together in the national interest on Brexit.

But Mr Blair suggested Labour should vote down whatever deal she brings back to Parliament in the hope of forcing a second referendum.

He said the Prime Minister was caught in a dilemma between a soft Brexit which would not match the hopes of Leave voters or a more decisive break from Brussels which could hit the economy.

“I would advise them to hold firm against Brexit because either of these choices are unpalatable,” he said at an event in London.

The DUP has said it will not accept a deal that treats Northern Ireland differently from the rest of the UK.

In a move seen by some as a warning shot, its MPs failed to back the Government in voting against a Labour amendment to an Agriculture Bill outlining post-Brexit reforms on Wednesday night.

Despite their abstention, it was still defeated by 59 votes.

The party’s Brexit spokesman Sammy Wilson later explicitly warned the Government against doing a deal that kept Northern Ireland in the single market, telling the Telegraph it could not support “any deal which includes such economically and constitutionally damaging arrangements”.

He added: “If the Government decides in the face of EU belligerence to cut and run and leave part of the UK languishing in the stifling embrace of the EU, then that would be totally unacceptable to us and many others in the House of Commons.

“It would have implications not just for Brexit legislation, 50% of which would not have passed without DUP support, but also for the Budget, welfare reform and other domestic legislation.”

Labour leader Jeremy Corbyn said the DUP’s warning was a “very real threat” to the Government and criticised the PM for failing to make progress on the Irish backstop.

“Quite simply, there has to be open trade between Northern Ireland and the Republic of Ireland and that has to be the basis of any agreement,” he added.