David Cameron has joined forces with a former trade union leader to warn that leaving the EU would be a "disaster for working people".
The PM set aside his political differences with ex-TUC boss Brendan Barber to pen a joint article for The Guardian, angering trade unions.
The pair claimed jobs would be lost and wages hit by the "shock" of an UK exit.
Downing Street has denied watering down new trade union laws to get the movement on side.
The UK will vote on 23 June on whether to remain part of the EU.
Leave campaigners claim quitting the EU will give a shot in the arm to British businesses hampered by red tape from Brussels - particularly as most small businesses do not trade with the EU but still have to abide by its regulations.
Employment minister Minister Priti Patel, who is campaigning for a British exit, said it would mean the UK could introduce one of the most flexible and liberating regulatory regimes anywhere in the world.
But the Remain camp says an exit could lead to the destruction of workers' rights.
Labour leader Jeremy Corbyn has warned the Conservatives would carry out a "bonfire" of rights if Britain voted to leave - a point echoed by the major trade unions.
In their article, Mr Cameron and Sir Brendan Barber argue that leaving the EU would risk jobs, hold down wages and lead to higher prices.
On the issue of workers rights, they say: "While the two of us may disagree about quite how far this process should go, being in Europe has helped to deliver many of the crucial rights that underpin fairness at work.
"Paid holidays, maternity rights, equal treatment for the millions of people working part-time, protections for agency workers, even equal pay for women at work: all are guaranteed by Europe and all could be at risk if we left."
They say "very special circumstances" have brought them together, adding that they are "united in our conviction that Britain - and Britain's workers - will be better off in a reformed Europe than out on our own".
Who is Sir Brendan Barber?
- Led the TUC from June 2003 until his retirement at the end of 2012. At 65, he now chairs the Advisory, Conciliation and Arbitration Service (ACAS) Council
- Joined the TUC in 1975 as training policy officer, moving up through the ranks to become head of press during the miners' strike and the Wapping dispute, then to run the department responsible for sorting out inter-union rows
- Appointed TUC deputy general secretary in 1993 when John Monks, now Lord Monks, stepped up to the top job. He took Lord Monks' place 10 years later
- Knighted for his work in 2013 as "the most visible ambassador and champion for the trade union movement over the last decade"
- Studied social sciences at City University, London
- Married with two daughters, lives in north London and supports Everton
Pressure on the pound would also lead to higher prices, they claim, saying this is a "risk working people and the poorest in our country simply cannot afford".
But Fire Brigades Union leader Matt Wrack said Sir Brendan's intervention with Mr Cameron was "appalling" and he accused the former union boss of "letting us all down" by joining forces with the prime minister.
"We are staggered that Sir Brendan Barber has lined up with him in this way," he said. "Nobody in the trade union movement can seriously believe that Cameron has the slightest interest in protecting workers' rights."
The joint Guardian article comes after ministers were accused in the Commons of making concessions in their Trade Union Bill in order to win union support for remaining in the EU.
'Sale of government policy'
After changes including a review of e-voting in strike ballots were unveiled, former Labour minister Kevan Jones said the "climbdown" on trade unions reflected the PM's "realisation" that he would "have to keep them onside".
Downing Street said the EU referendum was "a separate issue", saying many unions had already set out their views on the subject.
But pro-exit Tory MP Bernard Jenkin said he had been told No 10 had offered concessions "after discussions with trade union representatives", adding that, if true, it would amount to the "sale of government policy" and show that "this government really is at the rotten heart of the European Union".
Vote Leave, meanwhile, has produced what it calls a "new dossier" which it says shows how the EU's single market is "failing" small businesses.
A group of eight influential economists have also thrown their support behind the Leave campaign, arguing that leaving the EU would boost the UK economy by 4% in 10 years.
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