Tory MPs who supported the UK remaining in the European Union have been challenged by the Liberal Democrats to reject Theresa May's "hard Brexit" plans.
Lib Dem leader Tim Farron said the election was a "litmus test" for pro-EU Tories, and urged them to refuse to support the Tory manifesto.
The party warned that Tory MPs in Remain-supporting areas faced a "vicious backlash" in the election unless they campaigned to keep the UK in the single market and within the jurisdiction of the European Court of Justice (ECJ).
The Lib Dems also challenged pro-EU Tories to support the right to remain for European citizens, workers' rights, consumer protection and environmental measures.
The Prime Minister has already made clear she wants to leave the single market and end the jurisdiction of the ECJ as part of her Brexit strategy, so it is highly unlikely that Tory candidates will sign up to Mr Farron's list of demands.
The Lib Dems point to the by-election success in Richmond Park - where former Tory Zac Goldsmith was defeated as an independent - as an indication that Remain-supporting Tories will switch to Mr Farron's party.
Mr Farron said: "If you want to stop a hard Brexit and keep Britain in the single market, you need MPs who will fight for you.
"These are things thousands of voters hold dear and things that the Liberal Democrats believe with every fibre of our being.
"This is a litmus test. If these Tory MPs reject these principles and back a hard Brexit manifesto then people will know that on the biggest issue of the day they went missing in action.
"People will then know only the Liberal Democrats will fight for a country that is open, tolerant and united."
A Lib Dem source added: "Remain Conservative MPs cannot sign up to a hard Brexit manifesto, it goes against everything they say they believe.
"They need to stand up for what thousands of their voters in their seats believe in. Otherwise they will face a vicious backlash in the election - we know these voters are open to the Liberal Democrats."