A group of Labour figures have called on the party to form an electoral pact with the Green Party in a progressive alliance to guard against the looming "danger of a Tory landslide".
Former shadow cabinet minister Clive Lewis, musician Billy Bragg and ex-policy chief Jon Cruddas were among those calling on Labour to stand aside in two seats where it has "no realistic hope of winning" to extract a similar favour from the Greens in "a swathe of other seats".
They said Labour should not field candidates in Brighton Pavilion, held by Green Party co-leader Caroline Lucas, and the Isle of Wight, where incumbent Tory MP Andrew Turner is standing down after reportedly telling a group of A-level students that homosexuality was "wrong", and where the Greens finished third in 2015.
Two of the three MPs who signed the open letter appear to have a direct interest in an electoral pact with the Greens.
Mr Lewis won in Norwich South in 2015 with a majority of 7,654, slightly greater than the Green vote of 6,749.
And Tulip Siddiq won in ultra-marginal Hampstead and Kilburn with a majority of just 1,138, less than the 2015 Green vote of 2,387.
Their letter in the Guardian, also signed by Labour peers Baroness Kennedy of The Shaws and Baroness Lister of Burtersett, said: "As Labour members and supporters we believe the party is the best vehicle to create the kind of more equal, democratic and sustainable society we want to see.
"But with the progressive vote split, the danger of a Tory landslide and all it means for our country now looms darkly on 8 June.
"It is therefore important to maximise progressive votes and campaigning in some key seats.
"In that respect we applaud the move made by the Green Party in Ealing Central and Brighton Kemptown to stand aside and give Labour a better chance of defeating the Tories.
"If Labour were to respond, not only would it be doing the right thing morally, it could unlock further positive moves from the Green Party and its supporters in a swathe of other seats.
"We therefore urge the Labour leadership not to stand candidates in just two seats, Brighton Pavilion, the one seat the Greens now hold, and the Isle of Wight, the one seat where they are the best-placed party to defeat the Tories. In both instances, Labour has no realistic hope of winning.
"This is both the right thing to do and helps Labour in seats where the Green vote can make the difference to our party winning or losing. Labour now has to give something back to gain even more."
Labour rejected the call for a progressive alliance.
A party spokesman said: "Labour is fighting to win this election and will field candidates in every seat, with the exception of the Speaker's on grounds of parliamentary protocol."