The Liberal Democrats have tabled an amendment to the Queen's Speech calling for Britain to stay the single market and customs union.
The party is now calling on both Labour and Conservative MPs to support the amendment and inflict a historic defeat on the government, forcing Theresa May to soften her extreme approach to Brexit.
An amendment to the Queen's speech last year was accepted by the government after it received cross-party support.
Liberal Democrat Brexit Spokesperson Tom Brake commented:
"We've already seen living standards fall and prices rise since last year's Brexit vote.
"But this is nothing compared to the pain that would be caused by Theresa May's extreme version of Brexit.
"Tearing up Britain's membership of the world's largest market would mean fewer jobs, higher prices in the shops and a long-term blow to our prosperity.
"We will not give up the fight to protect our economy by staying in the Single Market and Customs Union.
"I am urging all like-minded Labour and Conservative MPs to be on the right side of history and back this amendment."