All six living former Labour leaders have come together to issue a plea for the party's supporters to vote Remain in the EU referendum, warning: "If Labour stays at home, Britain leaves."
In an open letter released by Britain Stronger in Europe, Lord (Neil) Kinnock, Dame Margaret Beckett, Tony Blair, Gordon Brown, Harriet Harman and Ed Miliband warned that Labour voters cannot afford to opt out of involvement in what many see as an internal spat between Conservatives.
"Only Labour can save Britain from Brexit," they said.
Labour's traditional working-class supporters have "the most to lose if we leave, but also the most to gain if we remain", said the letter.
Meanwhile, leading Leave campaigners said quitting the European Union would pave the way for a £100 million a week "cash transfusion" into the ailing NHS.
Pressures on the straining service would be alleviated by diverting funding that currently goes towards Britain's contribution to EU coffers, according to Michael Gove, Boris Johnson and Gisela Stuart.
The Justice Secretary called on the Government to make the funding pledge in the event of Brexit just hours after he faced a live television grilling ahead of the June 23 poll.
During the Sky News setpiece he claimed "the majority of people in this country are suffering as a result of our membership of the European Union but admitted he "can't guarantee every person currently in work in their current job will keep their job".
Labour's former leaders said Brexit would present a "double threat" of a return of recession, along with a swing to the right as Conservative hardliners gain the upper hand in government.
They gave their backing to the current Labour leadership in urging "every person who seeks a progressive future for Britain" to vote Remain.
Jeremy Corbyn - who has faced accusations of lukewarm advocacy for EU membership - has not signed the letter, which a Stronger In spokesman said was intended only for former leaders.
Many observers believe Labour voters could hold the key to the outcome of the referendum, with Remain strategists concerned that a low turn-out by the party's supporters on June 23 could hand victory to the Leave camp.
While polls show the party's voters are significantly more likely than Conservatives to back Remain, they are also less likely to take part in the referendum - with a recent ORB survey finding 52% of Labour voters saying they were "certain to vote", against 69% of Tories and 71% of Ukip supporters.
Mr Brown argues that around seven to nine million "progressive" voters - from around 14 million who backed Labour, the Lib Dems, Greens and SNP in the 2015 general election - will need to back Remain if Britain is to stay in the EU.
Speaking at the Hay-on-Wye Festival on Saturday, the former PM is due to call for a "positive, principled and progressive" message to persuade these people to back Remain to support jobs, security and fairness.
"To win the referendum on June 23, the largest Remain vote - perhaps seven to nine million people, depending on turnout - has to come from non-Conservative voters," Mr Brown is expected to say.
"But recent polls suggest that a large number of Labour supporters may not support Remain but instead remain at home.
"In one recent poll, as many as 62% of skilled workers are at risk of voting Leave unless we send out positive messages on employment rights. Why? Because they feel economically insecure, they don't like the status quo and they need to know that Europe offers something better for their future."
The joint letter argued that the EU has delivered "significant benefits" for working people, including more jobs, protections at work, and lower prices.
If the UK leaves, these communities would suffer "spending cuts, neglect for the needy and a bonfire of workers' rights", the letter warned.
The six Labour leaders wrote: "Europe protects people at work; stimulates jobs and innovation; keeps prices lower; leads global action against climate change; makes us safer against terrorism; and magnifies Britain's voice and values in the world.
"But make no mistake: this would be lost if we leave. Labour communities would face a double threat: the return of recession, led by a Tory government with an emboldened right wing.
"In such circumstances Labour communities would suffer most: from spending cuts, neglect for the needy and a bonfire of workers' rights.
"Those Labour seeks to represent - the hard-working, ambitious majority - have the most to lose if we leave. But also the most to gain if we remain."
The letter said that within the EU, Britain will be able to lead and shape the world, spread opportunity in the single market, invest in its children and pursue its interests in a globalised world.
But it added: "This should be our future. But we need to vote for it. If Labour stays at home, Britain leaves. And a vote to leave is a vote for a profound and permanent loss the whole country would feel, whether through lost jobs or lost generations. Only Labour can save Britain from Brexit.
"We have each seen the benefits of Europe. 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.
"That's why we join with our present leadership in urging every person who seeks a progressive future for Britain to vote Remain on June 23."
Leave campaigners will hold a London rally championing the Brexit cause.
In a joint statement, Conservatives Mr Gove and Mr Johnson and Labour's Ms Stuart, said the number of migrants from within the EU arriving in the country last year amounted to a city the size of Newcastle.
"As our population grows, and as we all live for longer, so the pressures on the NHS are set to grow," they said.
"We believe that one of the best ways to protect, and to strengthen, the NHS, for the people of this country is to use some money we currently spend on EU membership to invest in improving healthcare."
"After we Vote Leave on 23 June, the Government should use some of the billions saved from leaving the EU to give at least a £100 million per week cash transfusion to the NHS," the trio added.
"This money will be over and above the commitment that the Prime Minister rightly made at the last election to an £8 billion real terms increase."