Frances O'Grady, the new head of Britain's biggest trade union, will warn in a speech that Prime Minister David Cameron's "sinister" agenda in promising a referendum on Britain's membership in the European Union is to try and "repatriate" workers' rights from Europe.
In a Madrid speech to the European Trade Union Federation conference, UK TUC General Secretary O'Grady will urge fellow Europeans to reject attempts to withdraw Britain from the EU as it would undermine all workers' rights across the 27-member state.
"As well 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," O'Grady will tell conference delegates according to details of the address published on the TUC's website.
"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."
Cameron has promised an in/out referendum on the UK's membership of the EU by 2018 as he looks to the 2015 general election, where he will try to secure the Tory majority he failed to achieve in 2010, which forced him into a coalition with the Liberal Democrats.
Both his coalition partners and the opposition Labour Party are against holding a referendum on the EU, arguing it will create uncertainty for business at a time when the economy desperately needs confidence and investment to help it to a recovery.
However, the EU's critics insist the UK gets a raw deal from its membership of a union they say is bureaucratic, expensive and unaccountable.
Under EU laws, workers are given a number of protections and rights.
For example, the Working Time Directive entitles workers to rights such as minimum periods of daily and weekly rest, as well as annual leave.
"Some of your governments will be tempted to let David Cameron have what he wants, just to keep Britain in the EU. And some will be tempted to pull the very same trick and get rid of their own workers' hard-won rights too," O'Grady is set to say.
"British working people are looking to their colleagues around Europe to work with us. Trade unions are all about solidarity, about working together in the common interest. We must make common cause to defeat David Cameron's attack on working people and Social Europe."