Brighton Pavilion MP Caroline Lucas is calling on Labour and the Liberal Democrats to co-operate with the Green Party in key seats to defeat the Conservatives.
Ms Lucas and Jonathan Bartley, co-leaders of the Greens, have written to Labour leader Jeremy Corbyn and Tim Farron, the Lib Dem leader, setting out their case.
They wrote: “Greens have a powerful and compelling vision for building a better, bolder Britain and, like you, will be using the election to set out our policies and ask for voters’ support.
“However, we also continue to believe there is a role for some form of co-operation in a handful of seats to create the best possible chance of beating the Tories and, crucially, of thereby delivering a fairer voting system.”
Ms Lucas said: “Britain is at a crossroads and this election will dictate the very future of our country.
“The Green Party will be standing on a unique policy platform – opposing the Tories’ Brexit and putting forward big ideas for a fairer economy and the protection of our environment.
“Our call for a meeting between party leaders isn’t about the Greens standing aside – it’s about giving people in this country the best possible chance of defeating the Conservatives and bringing in a truly democratic voting system.
“For the sake of our NHS, our welfare state and our environment we need progressive party leaders to ditch partisan politics just for a moment and think about how we can best stop the Tories from wrecking our country for generations to come.”
In response Simon Kirby, the Conservative MP for Brighton Kemptown, said: “The Green Party can do what they like but it’s a real privilege to stand for Parliament and I welcome as much choice for local people as possible.”
Peter Kyle, the Labour MP for Hove, said: “I do not believe that there is a public appetite for a progressive alliance.
“I desperately want to oust this Tory government, which has time and again put the interests of the Conservative Party above the interests of our country, and I believe that the Labour Party is the only Party that can achieve this.
“The Labour Party is the single greatest vehicle for social change that this country has ever seen and I am proud to stand on a Labour ticket once again.”
At a Sussex Progressives meeting in Brighton last week Mr Bartley spoke about electoral reform. Other speakers included Klina Jordan, from the Make Votes Matter campaign group, the Labour MP Stephen Kinnock and the Lib Dem MP Tom Brake.
Mr Bartley said that Britain was looking down the barrel of decades of Tory rule. He said that he was frightened for his community which led him to the conclusion that when you have common ground, you work together.
He said: “It is the desperation of decades of Conservative rule that is bringing people together.”
Last year the Greens stood aside in the Richmond Park by-election to try to minimise any split in the vote against Zac Goldsmith.
The Conservative MP resigned over the decision to give permission for a new runway at Heathrow, prompting a by-election in which ran as an independent.
He was beaten by the Lib Dem Sarah Olney by just under 2,000 votes. At the previous election more than 3,500 people voted Green.