PENNY Mordaunt has conspicuously failed to support Theresa May’s Brexit plan as a leading Brexiteer claimed there were at least 40 Tory MPs ready to vote it down at Westminster.

The pro-Brexit International Development Secretary repeatedly refused to back the Prime Minister’s Chequers blueprint for EU withdrawal, adding pressure on Mrs May to give ground ahead of the crunch European Council next week.

Downing Street said that the UK Government’s new proposals on custom regulations regarding the Irish border would be published in due course. When asked if this meant it would be before the PM attends next week’s summit, her spokesman replied: “It means in due course.”

This morning after making a speech on aid, Ms Mordaunt was asked several times whether or not she supported the Chequers Plan and each time she dodged the question.

Rather, the Secretary of State said she would not give a “running commentary” on the proposals, saying: “The Prime Minister has my full support. She is working absolutely flat-out to get our country the best deal possible.

“I don’t doubt her motives, I don’t doubt her commitment and I don’t doubt for one moment her understanding that we have to deliver a good Brexit; we have to honour that result. So, she has my support and I am not in any way expecting that situation to change.”

Ms Mordaunt went on: “The Prime Minister can count on my support. But what I would say is that we don’t know where this is going to end up. We are at a critical moment now. The ball is firmly back in the EU’s court; we are waiting for them to respond.”

She added: “All that matters is where we end up, what that agreement is and I am going to do everything I can to ensure that is the best possible.

“I feel very strongly that we must honour the result of the referendum, honour the expectations of the British public in that.”

Mrs May chaired the weekly Cabinet at which she told colleagues the Government’s focus in the final stage of the talks with Brussels was on the few remaining issues, including the Northern Ireland backstop.

Her spokesman said the PM had explained: “'We cannot agree to a withdrawal agreement without a precise future framework which delivers the future trade and security partnership that the Government wants.’”

Asked about Ms Mordaunt’s failure to openly back the Chequers Plan, he replied: “What I heard Penny Mordaunt say this morning was that she was behind the Prime Minister and she expected that it would remain that way; that’s true of all Government ministers.”

Earlier, Steve Baker, the former Brexit Minister, said he stood by his previous claim that 80 Conservatives would be prepared to oppose Mrs May if her deal with the EU kept Britain too closely aligned to it.

The Buckinghamshire MP, a senior figure in the hardline Brexiteer European Research Group, said he was expecting the Tory whips to go to work on the rebels but that half of them would stand firm against her regardless.

He said that they would “not tolerate a half-in, half-out Brexit" after reports that the PM hoped to break the deadlock over the Irish border by keeping the EU's present customs arrangements beyond when the transition period is due to end in December 2020.

If 40 Tories rebelled, it would leave a Brexit deal at serious risk of failing to gain parliamentary approval, with Mrs May needing Labour votes to get it through.

"I always try to be accurate on the numbers rather than have a bluff to be called,” said Mr Baker. “We are in a position where, as we roll forward, colleagues will not tolerate a half-in, half-out Brexit.

"I did a concrete canvas of colleagues when it was amendments to legislation and came up with the number of nearly 80.

"Of course the Government are going to whip this vote extremely hard but what I would say is that the whips would be doing incredibly well if they were to halve the numbers and my estimate is that there are at least 40 colleagues who are not going to accept a half-in, half-out Chequers deal or indeed a backstop that leaves us in the internal market and the Customs Union, come what may."

Meanwhile, Mark Harper, who was a Home Office Minister during Mrs May's tenure in the department and later the Government Chief Whip, warned her that relying on Labour rebels would not work, saying: "We are going to have to carry this deal on our own benches. If you're the Prime Minister, you do have to listen to colleagues."