The new leader of the TUC will appeal for help from unions across Europe to persuade their governments to resist any attempt by David Cameron to "repatriate" workers' rights.
General Secretary Frances O'Grady will tell a conference in Madrid that if the Prime Minister gets his way over Europe, British workers, as well as those across the continent, will lose out.
She will say that Mr Cameron was putting internal party management above the national and European interest by pledging to hold a referendum on British membership of the European Union if he wins the next general election.
"What David Cameron is doing is even more sinister. As well as bringing the prospect of an unprecedented triple-dip recession even closer, the UK government is making the most vulnerable pay for a crisis they didn't cause, and is set on a wholesale scrapping of workers' rights.
"The Government has already made it easier for employers to sack people they don't like and more difficult for workers to get justice before the courts. Now it is trying to abolish wage protection for farm workers, and stop people injured at work getting their rightful compensation.
"But there's one set of workers' rights David Cameron can't touch. Those are the rights provided for by social Europe - paid holidays, health and safety, equal treatment for part-time workers and women, protection when a business is sold off, and a voice at work.
"The Prime Minister wants to repatriate those rights, and not because he thinks he can improve them. David Cameron wants to make it easier for bad employers to undercut good ones, drive down wages, and make people who already work some of the longest hours in Europe work even longer. To do that, he needs agreement from the rest of Europe."
Ms O'Grady will tell the event, marking 40 years of the European Trade Union Confederation, that unions had to make the case for a worker's and citizen's Europe, not a banker's and financier's Europe.