A Welsh Conservative MP has said he had no choice but to resign over Theresa May's concessions to Brexiteers.
Guto Bebb resigned as minister for defence procurement to vote against the government on amendments it accepted to its Brexit Customs Bill on Monday.
He told BBC Wales the "wrecking amendments" had undermined Mrs May's position in negotiations with Brussels.
Welsh Secretary Alun Cairns said he was "very sorry" the Aberconwy MP felt he had to quit over the issue.
The government avoided defeat by three votes on two amendments, and was accused of "caving in" to the Tory party's Eurosceptic MPs.
Downing Street accepted four amendments altogether, saying they were "consistent" with the government's position on how it wanted to trade with the EU in years to come.
On Tuesday, Mr Bebb said Brexiteer's "wrecking amendments" had been put forward "with a simple intention of damaging the Chequers agreement", the blueprint for future relations with the EU the prime minister agreed with cabinet earlier this month.
"When the government were not willing to compromise on this issue I felt that I had no choice because ultimately I wanted to send the prime minister to talk to the European Commission, to negotiate with the European Commission, from a position of strength with the party united behind her," he said.
"I think the ERG [pro-Brexit Conservative European Research Group] amendments yesterday have absolutely undermined the negotiating position that the prime minister had and I felt duty bound to try and vote to highlight the fact that the Conservative Party should be led by the prime minister not by the leadership of the ERG group.
"I'm disappointed that that point wasn't made last night with a victory in the vote in question."
MPs backed an amendment preventing the UK collecting taxes on behalf of the EU unless the rest of the EU does the same for the UK.
Applying EU tariffs to products destined for the EU is part of Mrs May's plan to avoid friction at UK borders after Brexit.
Another amendment, to ensure the UK is out of the EU's VAT regime, was backed by 303 to 300.
Mr Bebb was appointed minister for defence procurement in January 2018 after serving as parliamentary under secretary of state at the Wales Office and a government whip from March 2016.
Following his departure from government, he warned his whole party could go the same way if its divisions continued.
"I think the Conservative Party needs to think very carefully as to what will be the impact on the United Kingdom if we ended up with a Marxist Labour government intent upon some of the policies which are being supported by Jeremy Corbyn and his Labour leadership," he said.
"There are some in the party who are more intent on the concept of ideological purity on a Brexit which most people didn't vote for.
"Most people voted to leave the European Union not for a termination of our relationship with the EU."
Mr Bebb had staunchly defended the prime minister amid the turmoil which has followed the Chequers agreement, branding Boris Johnson's resignation as foreign secretary "a selfish act" and attacking former Brexit Minister David Jones for criticising Mrs May's plan.
Earlier on Tuesday Mr Cairns told BBC Radio Wales he was "going to miss" Mr Bebb, who had served under him as a junior Wales Office minister.
Speaking on Good Morning Wales, he said: "Guto was an excellent minister and a close colleague and a good friend.
"I'm very sorry that he felt in order to vote last night that he needed to leave the government."
Clwyd West MP David Jones - a former Brexit minister and Welsh secretary who led the Vote Leave campaign in Wales - said the government was right to listen to its Brexiteer critics but believed "a lot more listening" needed to be done.
Analysis - Elliw Gwawr, BBC Wales parliamentary correspondent
Guto Bebb told me he was increasingly concerned about the power of Brexiteers over the prime minister.
He believes the government bowed to pressure when it didn't necessarily need to, which leads him to question who actually leads the party now, Theresa May or a hard-core group of Brexit supporters.
The government insists that the amendments change nothing, but Mr Bebb told me that was "nonsense" and he now had real concerns about Mrs May's ability to negotiate a deal with the European Union.
The party's internal feud over Europe is worrying the former minister, who has serious concerns about the party's future if members don't become more pragmatic in their outlook about Brexit.