Usdaw, one of the UK's largest trade unions, today announced it would be backing British membership of the EU at its conference, a day after the CWU union committed its support for the same cause.
"If we were to leave the EU, there is a clear risk that such employment rights and protections would be dismantled by a UK Conservative government. Usdaw will always campaign to protect employment rights. However, this campaign will be best served through remaining in Europe," said John Hannet, Usdaw general secretary.
"The European Union is an international stage which we should be engaging with and not withdrawing from. Worker’s rights, jobs and our place in the world are just three of the many reasons why we should be voting to remain in the European Union."
Read more: Another union joins the Remain camp
The news also comes after Unison officially backed Remain ahead of the referendum earlier this month, following Unite and the GMB union.
Labour MP Alan Johnson, leading his party's Remain campaign, said: "Almost four million trade union members in every trade and sector, in every corner of Britain; from nurses and builders to steel workers, bus drivers, postal workers and shop workers; campaigning for Britain to remain in Europe.
"Campaigning for a Europe that protects working people and keeping the swivel-eyed alliance of the right of the Tory Party, and UKIP, away from hard won workers’ rights."
The development will be seen a further boost for the workers' rights argument for staying in the EU, especially in light of Labour leader Jeremy Corbyn's backing of membership.
"Being part of the Single Market, our home market of 500 million people, means more jobs, lower prices, and more financial security for British workers. And EU law protects crucial rights at work, including health and safety rules, maternity and paternity leave, and paid holidays," said Will Straw, executive director of Britain Stronger in Europe.