Dozens of Labour MPs have written to Theresa May warning her against putting Britain through an extreme Brexit if no good deal from the EU is forthcoming.
The prime minister has said she will walk away from negotiations if Brussels insists on playing hardball and will default to a low-tax, low-regulation stance to boost the UK's competitiveness.
But the group of MPs warn such a move would make Britain "the sweatshop of Europe" and leave public services, workers' rights and environmental protections all at risk.
Signatories to the letter 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.
The letter was organised by senior Labour MP Chuka Umunna, who chairs Vote Leave Watch.
If the government loses an appeal in the Supreme Court next week MPs will have to vote to approve a Brexit deal.
Mr Umunna said that while he would not vote against triggering Article 50, he would take a firmer line on the Brexit deal itself.
"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," he said.
"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."
Labour leader Jeremy Corbyn has insisted the referendum result must be respected and that he would ask Labour MPs to vote in favour of triggering Article 50 of the EU treaties.
But some 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.