Trade union leaders have warned that pulling out of the EU would allow the Tory government to erode workers' rights, issuing a plea to their members to vote to remain in the EU.
In a letter in The Guardian, the general secretaries of 10 of the UK's trade unions said that leaving the EU would put hard won workers' rights, including maternity and paternity rights, at risk.
"After much debate and deliberation we believe that the social and cultural benefits of remaining in the EU far outweigh any advantages of leaving," they said.
"We simply do not trust this government if they are presented with an unrestricted, unchecked opportunity to attack our current working rights."
Signatories to the letter include Len McCluskey of Unite and Dave Prentis of Unison.
Separately, activists from the GMB union - whose general secretary, Tim Roache, is one of the signatories to the letter - are expected to issued a "hands off Jeremy Corbyn" message to Labour MPs accused of undermining the party leader.
Mr Corbyn, who backs remaining in the EU, received a rousing reception when he addressed delegates at the annual conference of the GMB in Bournemouth.
A motion before the conference "deplores" Labour MPs who are "deliberately sabotaging" the work of the Opposition.