Labour has almost no chance of winning a majority at the next general election, according to analysis by a think tank.
The Fabian Society, which is strongly associated with the New Labour movement led by Tony Blair, said it is unthinkable that the party will win enough votes to govern alone.
It said the party must try to gain enough seats to form a centre-left coalition with other parties.
Its analysis of polling and election data suggests that Labour is likely to win between 140 and 200 constituencies on as little as 20% of the vote, which would be a further retreat from the 231 seats it currently holds.
And the party must position itself in the centre-ground, as it is losing as many votes to the pro-EU Liberal Democrats as Ukip and the Tories, meaning it must find a way to appeal to both Remain and Leave voters in a political landscape now defined by Brexit.
But the "firebreak" of the UK's first-past-the-post electoral system means it will still have a platform on which to rebuild.
The think-tank recommended that it should consider calls to form an alliance with the Lib Dems and SNP or other centre-left parties as the UK's political system continues to fracture.
Fabian Society general secretary Andrew Harrop said: "As things stand Labour is on track to win fewer than 200 seats, whether the next election comes this year or in 2020.
"Even if Labour recovers it has almost no chance of securing a majority in a general election, because it needs over three million more votes than the Conservatives to win.
"Labour's aim for now should be to move forwards not back and win enough MPs to be able to form a governing partnership with other parties."