The opt-out clause in the Working Time Directive, which allows employees to choose to work more than 48 hours per week, is under threat after Members of European Parliament (MEPs) and a UK Labour MEP voted to axe the deal.

The decision by the employment and social affairs committee will cut the working week to a maximum 48 hours. Earlier this year, the UK reached an agreement with other EU member states to support the Agency Workers Directive, giving temporary workers the same pay as permanent staff after 12 weeks in a job, in return for being allowed to keep its opt-out from the 48-hour week.

Liz Lynne, liberal democrat MEP, has criticised the UK Labour MEP who voted with socialist MEPs from other EU member states to axe the opt-out in the first stage of the parliament's review of the agreement. "The vote plays politics with people's pockets and runs the risk of sinking the government's hard won deal to retain the opt out," she said.

"Scrapping the opt out would be a bitter pill to swallow for businesses and many hard working people who are tightening their belts and who want to boost their earnings in difficult economic times." However, the government has insisted it will "stand firm" in protecting the opt-out clause.

Business secretary Lord Mandelson said: "The decision by the employment committee is not surprising in the least. We will continue to stand firm to protect the opt-out to the Working Time Directive's 48-hour maximum working week, which is essential to Britain's labour market flexibility that has helped to create an extra three million jobs over the past decade. "People must remain free to earn overtime if they wish and businesses must have the flexibility to cope during busy times. To lose the ability to opt out will mean workers cannot boost their earning potential when some are already struggling with high food and energy prices."

According to the Guardian newspaper the Union officials believe an opt-out could lead to widespread abuse including, at worst, law breaking, and at best, confusion among businesses. The unions claimed it is far too easy for employers to pressurise staff to work more than 48 hours a week. The issue will be debated next month by the European Parliament.