Open Britain, European Movement and Britain for Europe have teamed up to launch a key seat strategy for the general election in a bid to oppose a so-called 'hard Brexit'. The groups' combined supporter strength is around 600,000, with activists in nearly all of the UK's 650 constituencies.

The cross-party initiative will help volunteers find seats where they can help a Conservative, Labour, Liberal Democrat or Green candidate.

"Election candidates of all parties should be demanding that a hard Brexit is rejected and making clear that they will reserve judgement on the outcome until they see whether we get exactly the same trade benefits, as David Davis has promised," said Labour peer Lord Peter Mandelson.

"Nobody has to take a position on this now: they just have to ask the right questions and keep an open mind about the answers."

The three groups have released their opening list of 20 target seats on Tuesday 25 April. The list is expected to evolve and be added to as the campaign progresses and further key seats emerge.

Labour's Kate Hoey, the MP for the London seat of Vauxhall, and former Conservative leader Iain Duncan Smith, the MP for Chingford and Woodford Green, are just two of the Brexit backers on the target list.

The groups have also drawn up a list of 20 pro-EU MPs so that activists can help their re-election campaigns. Green co-leader Caroline Lucas, the MP for Brighton Pavilion, and Labour's Mary Creagh, the MP for Wakefield, are included on the list.

The development come after former Labour prime minister Tony Blair urged voters to back Conservative or Liberal Democrat candidates at the 8 June election if they had open minds about the final Brexit terms. But the calls was met with backlash from Labour centrists, such as Chuka Umunna.

"Tony Blair is wrong to suggest, in any way, that voters should look elsewhere and form some anti-Brexit alliance," he said. "No ifs, no buts: voting Labour and maximising our position in Parliament is the best way to stop Theresa May's hard Brexit."