Theresa May is risking fresh Brexit humiliation after hardline Eurosceptics warned they may not back a last-ditch move to help her secure a fresh deal from Brussels.

Jacob Rees-Mogg and other senior members of the European Research Group (ERG) of backbench Tories stunned No.10 by announcing they would not vote for a new amendment seen as a lifeline for the PM’s own proposals.

The amendment, tabled by Conservative grandee Sir Graham Brady, attempts to redefine the so-called Northern Irish ‘backstop’ that many Tories believe will potentially tie the UK indefinitely to EU rules.

In a new bid to break the Parliamentary deadlock, May told a packed meeting of Tory MPs on Monday night that she would impose a three-line whip to support the Brady proposal.

May, who delivered a withering put-down to Boris Johnson during the gathering, said that Brussels needed a clear vote from MPs rather than just complaints about the backstop.

She also urged her party give her time to bring back a fresh Brexit package, revealing a new promise to make a fresh update by February 13, with a Commons vote soon after.  One source suggested the ‘meaningful’ vote could be the following day, on Valentine’s Day.

But veteran Eurosceptics including Rees-Mogg, Sir Bernard Jenkin, Sir Bill Cash and Mark Francois dismissed the amendment as too vaguely worded to be legally binding and suggested they would not back it.

Up to 20 hardcore Brexiteers may refuse to back the Brady plan, and with a clutch of pro-EU Tories set to oppose it too the PM is facing yet another Commons defeat. The group will make its final decision at 6pm on Tuesday, just before the crunch vote.

Jenkin told ITV: “I will not be backing the Brady amendment as far as I’ve decided so far…It’s vague because it’s meant to mean different things to different people.

“If people vote for that in the expectation some things are going to happen and then they don’t happen it will lead to more misunderstanding and disappointment, it won’t help at all.”

Minutes before May’s meeting, Rees-Mogg emerged from the ERG’s own meeting to announce that Brady’s amendment was being “over-egged”, saying: “I don’t think it changes anything.”

“The Graham Brady amendment gives conditional approval, so that is an issue. It doesn’t say what it would be replaced with.

“And Graham has said he could live with a [legal] protocol rather than changes to the [Withdrawal Agreement] text, whereas from our point of view there needs to be changes to the text.

“There is no move to support it…What matters is what the Government is going to do – is it going to go back to the EU and ask for the Withdrawal Agreement to be reopened?”

In the meeting, May faced down hardline Brexiteers to warn that they needed to give her something concrete to return to Brussels.

Boris Johnson confronted her to demand some ‘straight advice’ on how meaningful the Brady amendment would be for the issue of the Northern Irish border.

“We won’t know unless you support us, Boris,” the PM hit back.

May also told the meeting: “If we are going to have a reasonable chance of convincing Brussels of the seriousness of the views held in this parliament, we have to do more than talk about it, we have to vote for it.”

Tory backbencher Simon Hart claimed that the ERG was split three ways, with 20 of its MPs set to back Brady, 20 still deciding and 20 against.

One Cabinet minister told HuffPost after the meeting that the ERG were proving their real agenda was to leave on March 29 without a deal.

“Some of them won’t agree to anything. She’s bent over backwards to accommodate them. They just don’t want any deal at all.”

Another Cabinet minister, a Brexiteer, added: “Is the ERG a thing? Two things were clear from that meeting: we want a deal and we don’t want the backstop. If I can vote for this, I don’t see why everyone else can’t.”

Tory party chairman Brandon Lewis confirmed that the government would whip in favour of the Brady amendment, and against the ‘Cooper-Boles amendment’ which seeks to delay Brexit and extend Article 50, if no deal has been secured by February 26.

Lewis said the Brady amendment “allows the Prime Minister to give a very clear message around what Parliament wants, where the party is”.

“I would hope the ERG, when they look at this and actually look through the detail of what this gives the PM tomorrow, in terms of a meaningful vote that will come back later, (see) it is about giving a message to Europe about what can go through parliament in terms of dealing with the backstop issue and why that matters.”

Lewis added that ministers would not incorporate Brady into a government amendment because of the risk that Labour MPs in Leave seats would then be less likely to support it.

May’s other challenge is to prevent pro-EU ministers from quitting in order to vote for the Cooper-Boles attempt to delay Brexit.

A No.10 source said that the PM told the meeting that if the government doesn’t bring the second ‘meaningful vote’ on her revised Brexit plan back by February 13, she would make a statement and table an amendable motion on the status of her proposals.

Earlier in Brussels, EU deputy chief negotiator Sabine Weyand was withering about the Brady amendment and warned that a no-deal Brexit was now more likely.

“We need to have a majority that doesn’t just get agreement over hurdle of a meaningful vote by a narrow majority but we need to have a stable majority to ensure the ratification. That’s quite a big challenge. There’s no negotiation between the UK and EU – that’s finished.

“There’s a very high risk of a crash out not by design, but by accident. Perhaps by the design of Article 50, but not by policymakers.”