Fears have grown for the UK's cherished opt-out from the Working Time Directive, with claims that even Labour MEPs will vote against it at a crucial vote in the European Parliament in December.
Employers believed the UK had secured its exclusion by conceding ground on the Agency Workers Directive, which was passed last week.
But a spokesman for the European Parliament told Personnel Today: "Even Labour MEPs are not backing the UK position. There is a possibility that there will not be enough votes to maintain the opt-out. It could go to conciliation."
The UK's Working Time Regulations currently allow workers to put in more than the 48-hour average working week imposed elsewhere in the EU. But David Yeandle, head of employment policy at manufacturing employers' group EEF, said there was a real danger that the conciliation process could see that opt-out diluted.
"There could be a more prescriptive review clause, effectively setting a timetable for the end of the opt-out," Yeandle said.
He added that the MEPs were not party to the deal made between member states on agency workers and working time. "There is a real possibility that there will be enough of a majority in the European Parliament to block the progress of the directive," Yeandle said.
The Agency Workers Directive passed last week will give temps equal rights to permanent staff after 12 weeks with an employer.
The CBI originally campaigned for a much longer qualifying period, but consented to a deal with the TUC in May on the understanding this would help the UK government retain the working time opt-out.
Katja Hall, director of employment policy at the CBI, said: "The Agency Workers and Working Time Directives were linked as a package at the European Council meeting in June.
"While agency work has been put to bed at a European level, it is now imperative that the UK's working time opt-out is retained, especially when the economy is slowing and businesses are facing an uphill struggle."
A European Parliamentary Labour Party spokeswoman said its MEPs had not yet decided how they would be voting in December.