Jeremy Corbyn needs to get round a table with the Greens to discuss ways of developing a progressive alliance, its co-leader Caroline Lucas has said.
The Green Party of England and Wales is not taking on Labour in Ealing Central and Acton and Brighton Kemp Town.
And Ms Lucas says left and centre-left parties should get together to prevent "a massive Tory landslide".
But shadow health secretary Jon Ashworth dismissed the idea of what he described as "back room deals".
And the Conservatives have warned against any "coalition of chaos".
Ms Lucas, the Greens' only MP in the last Parliament, said there was "a huge amount of enthusiasm and energy" for deals between parties such as the Greens, Liberal Democrats and Labour to defeat the Conservatives in the upcoming election.
It comes as the Greens withdrew their candidate in Oxford West and Abingdon to help the Liberal Democrat Layla Moran stand against health minister Nicola Blackwood.
The Lib Dems have already stood aside in Brighton Pavilion for Ms Lucas, but Labour rejected a similar deal for the Isle of Wight, despite Greens standing down to help Labour in Ealing Central and Acton.
Ms Lucas, who is campaigning on the Isle of Wight on Monday, told BBC Radio 4's Today programme it was time for the left and centre-left to get together "and do grown up politics".
'Different electoral system needed'
She said it was "scandalous" that the Conservatives got 51% of the vote across the south east region, but ended up with 93% of the seats.
"What we ought to be doing is saying: 'How do we stop the Tories getting the kind of landslide that will allow them to impose not just an extreme Brexit, but extreme social policies, long term damage to our health, education and so forth?'" she said.
She claimed the Isle of Wight Greens were "best placed to challenge" the Conservatives, ahead of the Liberal Democrats and Labour with a candidate "that will fight for this chance in our electoral system".
"But for this to happen, we need Jeremy Corbyn to get round a table with us," she said.
"What's so disappointing is that he talks about doing politics differently and yet he's betraying the millions of people he represents by allowing them to be hit hardest by a massive Tory majority."
Ms Lucas argues that more than one million people voted Green in the 2015 general election, which under a different electoral system could have led to 24 Green MPs.
"What we're saying is it's time to do politics differently. People in marginal constituencies don't want parties on the left and centre-left to spend all of their time fighting each other and for the Conservatives to come through the middle."
But Labour's Jon Ashworth refused to entertain the idea of entering into discussions with the Greens and Lib Dems about some candidates standing aside to avoid splitting the anti-Tory vote,
"We're a national party and we have a responsibility to give every part of England, Scotland and Wales the chance to vote for a Labour candidate," he said.
"Politicians who try to do these backroom deals, never come out of it well."