Open Britain is targeting key Leave supporters in 20 constituencies while throwing its backing behind 20 MPs who have spoken out against a hard exit from the EU since the referendum.
But the cross-party group, which grew out of the Remain campaign, has lost high-profile Tory backers Anna Soubry, ex-attorney general Dominic Grieve and Nicky Morgan after 18 of the seats where activists will be sent to campaign against the sitting MP were Tory.
Just one of the constituencies where they would rally to support the incumbent is held by a Conservative.
In a statement on behalf of the three, former education secretary Miss Morgan said: “As long standing Conservative party members and MPs it is untenable for us to play any further role in an organisation such as Open Britain, which is advocating campaigning against Conservative MPs or candidates.
“I and we will not be doing so.”
Former minister Miss Soubry said: “This sort of blatant partisanship further divides our country when we must all come together.”
Open Britain, along with European Movement and Britain for Europe, will draw on its database of 600,000 supporters to target the crucial seats in its bid to stop a hard Brexit.
A spokesman for Open Britain said: “It is understandable that during an election campaign political differences between members of different parties become magnified.
“Open Britain represents activists from all political persuasions who want to campaign against a hard Brexit and for a continued close relationship with Europe.
“As a grassroots organisation, we’ve had thousands of emails from our supporters asking how and where they should campaign during the General Election and we are trying to direct them to where they can make the biggest difference in the fight against hard Brexit.
“There is no way of getting involved in an election like this without campaigning for or against members of one party or another, so it was always going to disappoint some within a cross-party organisation.
“Our key seats list will continue to include support for those willing to campaign against hard Brexit from across the political spectrum.
“It has been a pleasure to work with some fantastic Conservative MPs in recent months and our door will always be open after the election to any that want to work with us in the future.”
Richard Tice, the co-chairman of Brexit campaigners Leave Means Leave, said: “Open Britain have been in denial about the reality that Britain is leaving the EU. They need to wake up and embrace the democratic decision for Brexit.
“They have continually demonstrated how out of touch they really are. Resigning from Open Britain is the right thing to do.”
April 25th, 2017: Express