Three trade unions opposed to the UK remaining in the EU argue it is a "myth" that membership benefits workers' rights.
The Rail, Maritime and Transport union, Aslef and the bakers' union BFAWU issued a joint statement explaining why they supported Brexit.
The statement said: "We support a Leave vote in the forthcoming referendum because we believe the EU acts overwhelmingly in the interests of big business and against the interests of workers.
"We note that David Cameron secured only very minor changes to EU rules and believe this demonstrates that there is little hope of reforming the EU."
The unions added: "It's a myth that the EU has won workers' rights and protections for workers.
"Nearly all the laws that protect workers in Britain are UK laws which have been won by the struggles and campaigns of the British trade union and Labour movement."
Most of the UK's biggest unions - including Unite, Unison, the GMB and CWU - have said they will support remaining in the EU.