A group of Conservative MPs will vote as a block against any Brexit deal based on Theresa May's Chequers plan, ministers have been warned.
Mark Francois told the BBC that he and other members of the European Research Group could not support the PM's plans as they "do not represent Brexit".
Asked how many MPs could rebel, he said it was "well north of 50".
Mr Francois said the PM would "lose" any vote and should embrace an alternative Canada-style trade plan.
Earlier on Tuesday, Labour said it was likely to oppose any deal negotiated by Theresa May as it did not meet its six tests, lessening the chances of it getting through Parliament.
Shadow Foreign Secretary Emily Thornberry told the party's conference the PM was proposing a "false choice" between a deal based on her Chequers blueprint for future co-operation - which would see the UK align closely with EU rules on trade in goods - and no deal at all.
The UK is due to leave the EU on 29 March 2019. Parliament will vote on any withdrawal deal, if it is agreed, before the end of the year or early next year.
Mr Francois suggested any deal based on Chequers had no chance of winning parliamentary approval and a looser Canada-style accord, which would not require the UK to sign up to EU rules, was what was needed.
"If it is based on Chequers then myself and my colleagues in the ERG cannot vote for Chequers because we believe it doesn't represent Brexit," the former defence minister told BBC Radio 4's World at One.
"If push really comes to shove and they try to put Chequers through the House of Commons then I and my colleagues will vote against it.
"In those circumstances the prime minister would then want to renegotiate along the lines of a free trade agreement, but far better that she pivots to that free trade agreement now before trying to take Chequers to the House of Commons and losing."
Although it is not clear how many MPs are currently affiliated to the ERG, 62 MPs signed a letter by the group in February urging the PM to revert to what they say was the vision of Brexit championed in her Lancaster House speech.
Former Brexit Secretary David Davis, who has indicated he will vote against the plans, said recently the ERG had about 30 to 40 members, with a core group of about a dozen.
No 10 insisted that ministers were united behind the PM's plan at a cabinet meeting on Tuesday.
Irish Prime Minister Leo Varadkar said there were no guarantees that any withdrawal agreement would be signed off by the UK and EU Parliaments.
"While we may assume that because something is agreed at European Council with the UK that it will go through both parliaments, we shouldn't actually assume it," he told the Irish Parliament.
"We're just going to have to manage that and see how it develops."