Six former Labour leaders are trying to rally the party's supporters to vote to remain in the EU.
Neil Kinnock, Tony Blair, Gordon Brown, Ed Miliband and ex-acting leaders Harriet Harman and Margaret Beckett, have issued a statement saying: "If Labour stays at home, Britain leaves".
They warned of a "bonfire of workers' rights" if the UK votes to leave.
Labour MPs backing an exit say workers' rights are the result of action by the UK, not the EU.
There is concern in the Remain camp that Labour is failing to energise its supporters to back their side.
Party leader Jeremy Corbyn - who has previously expressed Eurosceptic views - has been accused of being "half-hearted" in his backing for Labour's position to campaign to stay in the EU.
This week he said there was an "overwhelming case" to remain in, citing issues ranging from mobile phone charges to clean beaches and protecting bees - but also called for sweeping reforms.
The statement by his predecessors warns of a "double threat" to Labour-supporting communities if there is a Leave vote - of a recession and a "Tory government with an emboldened right wing".
They point to the UK's membership of the EU single market and say jobs would be lost if it leaves, adding: "Only Labour can save Britain from Brexit."
"More importantly, as those who have led Labour, we understand our party's values and its people. Each are strengthened by Britain being in Europe."
Their statement comes amid a drive to persuade people to register to vote ahead of the 7 June deadline.
In a separate speech Mr Brown will say Labour's campaign to stay in the EU is "now out in force".
He will say polls suggest the majority of skilled workers could vote to leave and claim the UK's European Council presidency in 2017 will lead to the "creation of thousands of new jobs".
One of the minority of Labour MPs backing Leave, Frank Field, recently warned the party it faced losing "a swathe" of voters to UKIP over its referendum stance.
Mr Field said Labour's traditional voters had seen their wages, housing and services hit by "open-door" immigration and said the party was not reflecting their views.