Theresa May's Brexit deal has been attacked as the "worst of all worlds" by one of her staunchest allies, who also said a trade deal warning by US President Donald Trump should not be ignored.
Former defence secretary Sir Michael Fallon confirmed he will not vote in favour of the agreement when it comes to Parliament, where he said it seems "doomed" to fail.
The MP for Sevenoaks, until now seen as a May loyalist, told BBC Radio 4's Today programme: "My fear is that this deal gives us the worst of all worlds – no guarantee of smooth trade in the future and no ability to reduce the tariffs that we need to conclude trade deals with the rest of the world.
"So, unless the House of Commons can be persuaded somehow that those are possible, then I think, yes, the deal is doomed."
And Mr Fallon said the Government must heed Mr Trump's claim on Monday night that the Withdrawal Agreement "sounds like a great deal for the EU".
He said: "It's no use us just brushing that off, saying 'No, no, we can do a deal with America'; he's the President of the United States, and if he says it's going to be difficult, then it certainly looks like it's going to be difficult.
"This is not a good deal and we need a better deal."
But Cabinet Office Minister David Lidington shrugged off Mr Trump's comments, telling the same programme: "I don't think it was that unexpected."
The PM's de facto deputy insisted the wording of the Withdrawal Agreement does mean the UK can still negotiate trade deals during the transition period, to be signed once the country fully exits the EU.
Responding to Mr Fallon, Mr Lidington said: "I think it was always going to be challenging to do a deal with the United States.
"The United States is a tough negotiator, President Trump's always said very plainly 'I put America first'.
"Well, I'd expect the British Prime Minister to put British interests first, but it's going to be a very tough negotiation."
I will take this deal back to the House of Commons confident we have achieved the best deal available and full of optimism about the future of our country. #BackTheBrexitDealpic.twitter.com/NBm2kGZU5B
— Theresa May (@theresa_may) November 25, 2018
It comes as Mrs May heads for Wales and Northern Ireland to try to sell the deal she brought back from Brussels on Sunday as "good for the union".
The PM also challenged Labour leader Jeremy Corbyn to a Brexit TV debate on the issue, as she launched an intensive two-week campaign to try to get her EU exit agreement through the Commons.
Mrs May told the Sun: "I am going to be explaining why I think this deal is the right deal for the UK – and, yes, I am ready to debate it with Jeremy Corbyn, because I have got a plan. He hasn't got a plan.
"Exactly how it might be done, if he takes it up, would be a matter for the broadcasters to determine.
"What I think is important is that people are able to see the issues around this plan.
"I am willing to stand up and explain why I think it is the best possible deal available for the UK."
A Labour spokesman said: "Jeremy would relish a head-to-head debate with Theresa May about her botched Brexit deal and the future of our country."
The comments came after Mr Trump appeared to undermine Mrs May's Brexit deal, telling reporters: "Sounds like a great deal for the EU.
"I think we have to take a look at, seriously, whether or not the UK is allowed to trade, because, you know, right now, if you look at the deal, they may not be able to trade with us. And that wouldn't be a good thing.
"I don't think they meant that. I don't think that the Prime Minister meant that. And, hopefully, she'll be able to do something about that.
"But, right now, as the deal stands, she may not, they may not, be able to trade with the US. And, I don't think they want that at all.
"That would be a very big negative for the deal."
In response, a Downing Street spokesman said: "The political declaration we have agreed with the EU is very clear we will have an independent trade policy so that the UK can sign trade deals with countries around the world – including with the US.
"We have already been laying the groundwork for an ambitious agreement with the US through our joint working groups, which have met five times so far.
"The US Trade Representative also issued a call for views from the public on a future UK-US free trade agreement earlier this month."