Dozens of Labour MPs have written to Theresa May condemning her plan to potentially put Britain through the hardest possible Brexit by threatening to leave the European Union with no trade deal.
The Prime Minister has insisted she will walk away from negotiations with Brussels if she is only offered a bad deal, threatening to adopt a Singapore-style low-tax, low-regulation economic model to maintain competitiveness.
The group of MPs have attacked Mrs May's idea, saying it would make Britain "the sweatshop of Europe" with public services, workers' rights and environmental protections all at risk.
The letter was organised by senior Labour MP Chuka Umunna, who chairs Vote Leave Watch and said he would not rebel against Jeremy Corbyn over triggering Brexit.
The Labour leader has insisted the referendum result must be respected and will ask MPs to vote in favour of triggering Article 50 of the EU treaties if the Government loses an appeal in the Supreme Court over the need for parliamentary approval next week.
Prominent Labour parliamentarians such as Lord Hain and Mike Gapes have promised to vote against triggering Article 50 and there have been suggestions that dozens of MPs in pro-Remain seats could rebel against the leader over the matter.
But Mr Umunna said: "Personally, as a democrat and having agreed to the rules under which the referendum was fought, I would find it hard to vote against triggering Article 50.
"But the content of the Brexit deal is a different matter - I am not prepared to give the Tories a blank cheque to make life harder for middle and lower income households in my constituency, a sentiment which is shared across the House of Commons."
The signatories include former deputy Labour leader Harriet Harman, two members of the current Labour frontbench, 10 former members of the shadow cabinet, and 15 MPs whose constituencies voted to leave the EU.
It comes as a cross-party group of MPs told the Observer they will amend any Article 50 Bill to make what they call Mrs May's "extreme Brexit" with no deal impossible.
Former deputy prime minister Nick Clegg told the newspaper he had been talking to MPs from other parties about how to gather support for amendments "because the situation is so serious we are condemned to work together on amendments that we can all support."
Meanwhile, shadow Brexit secretary Sir Keir Starmer said he would fight any attempts to water down human rights or environmental protections once the UK leaves the EU.
The so-called "Great Repeal Bill", which transposes all EU law into UK law, giving Parliament the power to decide which bits to keep, could include a "Henry VIII clause" allowing ministers to ditch or change sections through secondary legislation with minimal scrutiny by MPs.
That is because it the Bill is expected to pass before the Prime Minster concludes a final Brexit deal with the Government.
Sir Keir said Labour would fight against any attempts to push through changes without full parliamentary approval.
He told the Independent: "It would be wrong for these rights to go into our law and then be capable of being amended or removed by statutory instrument.
"I think that's a really important principle that we must fire to the beginning of any discussion on the Great Repeal Bill."