The Labour campaign to stay in the EU has been handed a boost after one of Britain’s biggest unions confirmed it will campaign for a Remain vote in the forthcoming referendum.

The union will not formally set out its position until the referendum campaign begins, which could be as early as this week, but incoming General Secretary Tim Roache has made clear that GMB will fight to retain EU membership.

Roache said that EU legislation is currently one of the few things standing “between our members and an all out assault from this Government”. Shadow Foreign Secretary Hilary Benn said the GMB’s decision was “great news for the campaign to be part of a progressive European Union”.

The announcement will come as a boost to the Stronger In campaign, as GMB has in recent years been among the most vocally Eurosceptic of the large trade unions. Both Unison and Unite have hinted that they may not campaign for a Remain vote – although neither have confirmed a position yet – and the GMB’s early move may be influential in getting other unions on board.

The strength of union support, including resources, could play a vital role in the referendum, with most polls showing a tight race.

Roache said:

“The EU isn’t perfect. GMB will continue to argue and campaign for a Europe that works for working people, not for big business, but we have to be realistic.”

He claimed that those arguing for a Leave vote do not care about the workers’ rights that are protected by EU legislation.

“Those who want us out of Europe don’t give a monkeys about protecting GMB members at work,” he said. “An ‘out’ vote would leave us at the mercy of a Tory Government that has systematically attacked our ability to organise and defend working people. EU laws currently stand between our members and an all out assault from this Government.

“Europe needs to change, but we make those changes by being inside the tent, not out. That’s what we’ll be saying to our members in the coming months.”

The announcement was met warmly by Hilary Benn, who celebrated the GMB’s recognition of “improved rights and protections that EU membership has given British workers”. He said:

“The announcement by the GMB that it will be campaigning to remain in the European Union is great news for the campaign to be part of a progressive European Union because it recognises the improved rights and protections that EU membership has given British workers, as well as the jobs, investment and growth that membership helps bring to Britain.”

Jeremy Corbyn is expected to travel to Brussels later this week, where he will condemn David Cameron’s proposal for an ’emergency brake’ on benefits for EU migrants.