Theresa May has been hit by a ministerial resignation as she prepares to hold talks with Jeremy Corbyn which the Government accepts are likely to lead to a "softer" Brexit.
Wales minister and whip Nigel Adams said the Prime Minister had made a "grave error" by reaching out to the Labour leader in the hope of finding a consensus solution to Brexit ahead of a crunch EU summit on April 10.
Mrs May announced her dramatic move after a marathon session of Cabinet at which a number of ministers are understood to have argued for a no-deal EU withdrawal next week.
In his letter, Mr Adams said that the Government faced two "great challenges" of delivering "the Brexit the people voted for" and preventing a Corbyn premiership.
Following yesterday's cabinet, this morning I've been to Downing Street & resigned my position as UK Govt Minister for Wales & Govt Whip. I'm grateful to the PM for the opportunity to serve as a Minister since 2017 & will continue to serve my constituents from the backbenches. pic.twitter.com/W15xS4cOBP
— Nigel Adams MP (@nadams) April 3, 2019
And he added: "Sadly, I fear that we are now at risk of simultaneously failing in both."
The Selby & Ainsty MP said: "Legitimising and turning to Jeremy Corbyn to assist you at this crucial stage, rather than being bold, is a grave error.
"It is clear that we will now end up in the customs union. That is not the Brexit my constituents were promised and it is contrary to the pledge we made in our manifesto. It makes no sense to leave the EU and to have a situation where our trade policy and much of our law is made in Brussels with no say for the UK."
Mr Adams, who was made a whip in January 2018 and promoted to the Wales Office in November, said he continued to believe that no deal was better than a bad deal.
"It now seems that you and your Cabinet have decided that a deal cooked up with a Marxist who has never once in his political life put British interests first is better than no deal, " he told the PM. "I profoundly disagree with this approach."
Mr Adams's resignation is the first since Tuesday's Cabinet meeting and came shortly after Brexit Secretary Stephen Barclay said a softer Brexit was the inevitable result of the political balance in the Commons.
Referring to a softer Brexit scenario, Mr Barclay told BBC Radio 4's Today programme: "It's undesirable, but it's the remorseless logic of the numbers of the House of Commons.
"The Prime Minister's deal won't go through and no deal in law is taken off the table, then the consequence of that is either a soft Brexit or no Brexit at all."
The Prime Minister's offer to meet the Labour leader to try and end the impasse over EU withdrawal sparked fury among Tory Eurosceptics.
But Mr Barclay blamed hardliners in Tory ranks for the situation.
He said: "It's regrettable that what we have been saying for several months now is coming to pass, but that is the remorseless logic of not backing the Prime Minister's deal.
"Because the alternative then is to have to seek votes from the opposition benches because 35 of my own colleagues would not support the Prime Minister's deal."
The comments came after Mrs May said she will seek an Article 50 extension beyond next week to allow negotiations with the Labour leader aimed at ensuring the UK leaves the European Union "in a timely and orderly way".
Mr Corbyn said he would be "very happy" to meet the Prime Minister in a bid to offer "certainty and security" to the British people – but Tory Brexiteers have reacted with anger.
Mr Barclay said he believed the Labour leader posed a threat to the UK economy, but he could work with him on Brexit.
The Brexit Secretary denied media reports that he was one of 14 Cabinet members to back a no-deal Brexit over the Prime Minister's latest plan in Monday's marathon Cabinet session.
Tory MP Jacob Rees-Mogg described the offer to Mr Corbyn as "deeply unsatisfactory" and accused Mrs May of planning to collaborate with "a known Marxist".
The DUP said it "remains to be seen if sub-contracting out the future of Brexit to Jeremy Corbyn, someone whom the Conservatives have demonised for four years, will end happily".
International Development Secretary Penny Mordaunt said: "The only way to leave is with the Withdrawal Agreement.
"The Prime Minister is trying to get it through on Conservative votes and has not been able to, she's now trying to do it with Labour votes."
The leaders' meeting is expected to take place as a cross-party group of senior MPs launches a separate attempt to force the PM to stop a no-deal Brexit by tabling legislation requiring her to delay Britain's withdrawal beyond April 12.
A group of MPs – including Conservative grandee Sir Oliver Letwin and Labour's Yvette Cooper – aim to fully pass a Bill through the Commons on Wednesday to stop the UK crashing out of the EU.
It follows the rejection of her Withdrawal Agreement three times in the Commons and failure of MPs to back any of the alternative proposals considered so far.
Instead of initiating a third round of indicative votes on Wednesday, when Parliament once more has control over the Commons timetable, Sir Oliver will table a paving motion to allow debate and votes on Ms Cooper's Bill. An amendment to his motion would set aside April 8 for indicative votes.
The single-clause Cooper Bill requires the Prime Minister to table her own motion seeking MPs' approval for an extension to the Article 50 process of Brexit talks to a date of her choosing.
The group behind the Bill hopes once it has passed the Commons it could be approved by the House of Lords and granted Royal Assent in time for the emergency EU summit on April 10.
Mr Rees-Mogg claimed Remainers had taken control of Brexit and were "disenfranchising Leave voters".
European Parliament Brexit coordinator Guy Verhofstadt, who had said he thought a no-deal Brexit was "nearly inevitable", welcomed Mrs May's offer of talks with Mr Corbyn.
If the European Council proposes an extension beyond May 22, it is understood that it would be possible for the UK to take the steps necessary to prepare for European Parliament elections on May 23, but then cancel them at the last minute if the withdrawal deal was ratified.