Campaigners have drawn up a hit list of Brexit-backing MPs they will try to oust at the General Election by flooding their seats with activists.
Key Leave supporters will be targeted in 20 constituencies while support will be given to 20 MPs who have spoken out against a hard exit from the EU since the referendum.
It comes as Labour sets out its Brexit strategy and the Tories continue to warn that Jeremy Corbyn's party is seeking to "disrupt" the negotiations.
Among the seats that pro-European groups Open Britain, European Movement and Britain for Europe plan to attack during the campaign are those of Tory former Cabinet minister Iain Duncan Smith and Labour's Kate Hoey.
It will also draw on its database of 600,000 supporters to support Labour's Mary Creagh, Conservative Neil Carmichael and Green MP Caroline Lucas.
Labour former Cabinet minister Lord Mandelson, an Open Britain board member, said: "As a former European trade commissioner, I have seen such negotiations from the inside.
"For Britain to get the best possible trade deal, it is totally counter-productive for Theresa May to go into them with a rigid set of red lines."
Labour will say retaining the benefits of the single market and customs union will become top priority in Brexit talks if it wins on June 8.
It would guarantee the rights of EU citizens living in the UK on day one of taking power without protections in place for Britons in Europe.
Shadow Brexit secretary Sir Keir Starmer will say Labour would ditch the Government's negotiating plans for quitting the bloc.
A Bill to transfer relevant EU laws to Britain would also be replaced with legislation that would protect employment and consumer rights as well as environmental protections when powers are repatriated.
The party will insist that failing to reach a deal with Brussels is not an option and will promise to give Parliament a meaningful vote on the final agreement.
Sir Keir will say: "EU nationals do not just contribute to our society: they are our society. And they should not be used as bargaining chips.
"So on day one of a Labour government we will immediately guarantee that all EU nationals currently living in the UK will see no change in their legal status as a result of Brexit, and we will seek reciprocal rights for UK citizens in the EU.
"There could be no clearer signal that Britain is taking a new approach to Brexit than a Labour government immediately rectifying this injustice. And there could no clearer signal that Labour want a close and collaborative future relationship with our EU partners."
Prime Minister Theresa May has said the UK "cannot possibly" remain part of the single market as it would mean "not leaving the EU at all", while leaders across the bloc have warned that Britain cannot enjoy the benefits without agreeing to the rules, such as freedom of movement.
Sir Keir will say: "The white paper will have a strong emphasis on retaining the benefits of the single market and the customs union as Labour know that is vital to protecting jobs and the economy.
"And we will approach negotiations in a completely different way to a Tory Brexit: negotiating for the many, not the few.
"Where Theresa May wants to shut down scrutiny and challenge, Labour will welcome it.
"We will work with Parliament, not against it. Because on an issue of this importance, the Government can't hide from the public or Parliament. A Labour approach to Brexit means legislating to guarantee that Parliament has a truly meaningful vote on the final Brexit deal."
Under Labour's Brexit strategy, the Great Repeal Bill would be replaced by an EU Rights and Protections Bill.
"This will make sure that all EU-derived laws - including workplace laws, consumer rights and environmental protections - are fully protected without qualifications, limitations or sunset clauses. We will work with trade unions, businesses and stakeholders to ensure there is a consensus on this vital issue.
"A Labour approach to Brexit will ensure there can be no rolling back of key rights and protections."