Labour is being urged to form a pact with the Green Party to ensure the Tories do not win in a landslide come 8 June.
Senior Labour figures, including former shadow cabinet minister Clive Lewis, musician Billy Bragg and ex-policy chief Jon Cruddas asked their party to stand aside in two seats where it has no realistic hope of winning, hoping the Greens would return the favour.
The news comes after the Green Party pulled out of crucial election seats in Ealing and Brighton Kemptown to help Labour MP Rupa Huq beat the Conservatives.
The two Labour seats at stake are in Brighton Pavilion, held by Green Party co-leader Caroline Lucas, and the Isle of Wight, where Tory MP Andrew Turner is standing down after reportedly railing against homosexuality to a group of A-level students and where the Greens finished third in 2015.
A total of 18 signatures were on the open letter, published by The Guardian, including three Labour MPs and Labour peers Baroness Kennedy of The Shaws and Baroness Lister of Burtersett.
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, the letter read.
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.
Prime Minister Theresa May has asked Labour voters to “lend her” their votes, and has insisted that she would provide a “strong and stable government”.
She was accused by critics of calling a snap election next month while she is ahead in the polls to consolidate her power.
Some surveys found the Tories’ lead over Labour has dropped by as much as 10 points in a week, but the lead is still around 17 points and points to a major victory in June.
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, the open letter continues.
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.
Mr Lewis won in Norwich South in 2015 with a majority of 7,654, just ahead of the Green vote of 6,749.
And MP Tulip Siddiq won in the north London constituency of Hampstead and Kilburn with a majority of 1,138, less than the 2015 Green vote of 2,387.
A progressive alliance” has been heralded by think tank Compass, which is encouraging people in constituencies across the UK to discuss collaboration with grassroots members of the three parties.
Stephen Clark, the organiser of West London Compass, previously told The Independent: Discussions are taking place in a number of the other seats between the Greens and the most likely potential progressive candidate, and we’re hoping something will come from those.
I think we will find as the election goes on that people will come this way. The system is crying out for it.
Labour has denied it will form a pact with other parties.
A party spokesman told The Independent: 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.