Britain's blue collar workers could turn their backs on an "In" vote at the promised EU referendum depending on David Cameron's renegotiation, the Trades Union Congress (TUC) has warned.
Frances O'Grady, the general secretary of the body, said that the prime minister could risk a Brexit if he limited workers' rights as part of his deal with Brussels.
"In the EU debate, as you know, the TUC is very clear: we're not going to win blue collar votes to stay in if people believe it's going to worsen their rights. If that's in the renegotiation package, that is a major problem for the forthcoming debate," said O'Grady.
The comments come as Cameron continues to hold "open and frank" talks with EU leaders as part of his plan to get a good deal for the UK.
But the prime minister suffered a blow after Charles Michel, the prime minister of Belgium, rejected Cameron's proposal to stop in-work benefits for EU citizens in Britain.
Michel reportedly said: "In Belgium we believe that the principle of free movement of workers is very important, that non-discrimination and equal rights for European citizens are very important."
O'Grady made the remarks at the Great Business Debate, hosted by the Confederation of British Industry (CBI) at law firm Linklaters today (12 June).
The trade union chief spoke alongside the CBI's director-general, John Cridland, who has thrown his support behind the "In" campaign.
But the discussion mostly focused on the UK's skills shortage, the economies productivity puzzle and a debate around real wages.
Cridland, for example, warned that there is a "fundamental mismatch" between living standards and the economic recovery in some parts of the country.
He said: "I think [the issue has] caught politicians where it's painful and it's caught the CBI and the business community where it's painful. What's the point of a really solid economic recovery if people in Lincoln can't smell it, touch it and be part of it?"
The CBI chief argued that the business community must be at the forefront of raising living standards and called on leaders to "sort education so that young people have better chances to get in the labour market".