Average UK wages would be £38 a week lower by 2030 if the UK votes to leave the EU, the Trades Union Congress says.
The pro-Remain organisation is also warning manufacturing jobs would go and employments right be put at risk.
TUC general secretary Frances O'Grady said trade and investment would be affected by an EU exit.
But another union chief, the RMT's Mick Cash, said the EU had pursued a "race to the bottom" on workers' rights and could never be reformed.
Publication of the TUC's research comes amid a drive to recruit traditional Labour voters by the rival sides ahead of the 23 June referendum.
It said the manufacturing sector would be hit seven times harder than those in the services sector because it exports so much to the EU.
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Ms O'Grady said: "Thirty eight pounds a week may not be much for politicians like Boris Johnson, but for millions of workers, it's the difference between heating or eating, between struggling or saving, and between getting by or getting on."
But Mr Cash - whose union represents people in the transport industry - said the EU was no "workers' paradise", pointing to industrial unrest in France and high unemployment rates in other countries.
He told BBC Radio 4's Today programme: "We as a union have opposed being members of the EU since 1979, because we do not think workers have benefited from it."