The minister who resigned over Theresa May's Brexit strategy says another referendum may be the only option if MPs reject the deal.
Science and universities minister Sam Gyimah quit after Mrs May pulled the UK out of the EU's Galileo sat-nav system, following a row with Brussels.
He said the decision showed the UK will be "hammered" in Brexit talks.
Mr Gyimah is the 10th person to resign over Mrs May's Brexit agreement. He said he intends to vote against it.
BBC political editor Laura Kuenssberg said it was "another very dangerous moment" for the prime minister as it meant not only that she had lost another vote for her deal, but also that others might share Mr Gyimah's concerns and quit her government as well.
"Mr Gyimah's departure shows it is not just Brexiteers who are unhappy and feel her deal ties us too closely to the EU for good - but also Remainers who are worried the deal is an unsavoury halfway house," she said.
MPs will vote on the Brexit agreement - which has already been agreed between the UK and EU - on 11 December. Labour, the Lib Dems, the SNP, the DUP and many Tory MPs have said they will vote against it.
Speaking on the Today programme on Saturday, Mr Gyimah - who voted Remain - said: "Looking at the deal in detail, we don't actually have a deal. We have a deal in name only.
"We have given up our voice, our veto and our vote. Our interests will be hammered because we will have no leverage."
He urged Mrs May not to rule out another referendum if she loses the 11 December vote.
"If Parliament was in deadlock, Theresa May could get herself out of that deadlock by backing a second referendum," he said.
Tory MP Mr Gyimah's resignation was announced on Friday night, shortly after Mrs May said the UK was pulling out of Galileo, which is Europe's version of GPS and due to be fully operational in 2020.
The UK wanted to stay part of it but the EU said it would be banned from extra-secure elements of the project.
Mr Gyimah said what happened with Galileo was a foretaste of the "brutal negotiations".
In a Facebook post setting out his reasons for resigning, Mr Gyimah said the UK's interests "will be repeatedly and permanently hammered by the EU27 for many years to come".
However, prominent Brexiteer and cabinet minister Michael Gove has defended Mrs May's plan, writing in the Daily Mail that leaving the EU is under "great threat" if the deal is rejected by MPs.
Will more Remainers voice their fears?
By Laura Kuenssberg, BBC political editor
As Theresa May was sitting down to at a glittering evening with her fellow world leaders at the G20, news broke that Sam Gyimah had just become the latest minister to quit over Brexit.
He had a specific reason to leave. But it is his overall verdict on Mrs May's Brexit compromise that will really hurt.
There is some comfort overnight for Mrs May from Michael Gove, who as one of the leading voices in the Leave campaign is, belatedly perhaps, urging his Brexiteer colleagues to get on board.
But this latest resignation is another sign of how hard it will be for the prime minister to pass the vote that could define her future.
Meanwhile, Mrs May is in Argentina, where she met Japanese Prime Minister Shinzo Abe for talks on the fringes of the G20 summit on Saturday.
He asked her to avoid a no-deal Brexit as major Japanese companies like Nissan and Honda are concerned about the possible impact on their supply chains across Europe.
She told him she was confident Japanese businesses in the UK would continue to trade well with the EU.
Mrs May also held talks with Australian Prime Minister Scott Morrison.
'Matter of compromise'
Culture Secretary Jeremy Wright said he was "very sad" to see Mr Gyimah leave the government and that he had been "a very good minister".
"All of my colleagues are going to have to make their own judgment about what they think about this deal," he told BBC Radio 4's Today programme.
He said that Mrs May's deal should not be compared with an idealised version of Brexit.
Former Tory cabinet minister and campaigner for a second referendum, Justine Greening, said Mr Gyimah was a "highly respected and capable minister" and praised him for not ruling out another vote.
And the Lib Dem's education spokeswoman, Layla Moran, said Mr Gyimah's exit showed the government was "falling apart", and that he had "seen at close quarters the devastating effect this botched Brexit will have on these important sectors".
Matt Waddup, head of policy and campaigns at the University and College Union, said: "This latest resignation shows that there are clear concerns about the impact of Brexit on research and innovation as well as upon higher and further education itself.
"Those concerns are shared by our members in colleges and universities who have clearly signalled that they want a referendum on any final deal."