Trade unionists say Britain's culture of working long hours must be curbed
The UK's right to opt out of the European Working Time Directive is to come under consideration in formal talks in Brussels.
The directive aims to limit the working week to 48 hours.
The government wants to keep the opt out, but in December members of the European Parliament voted to scrap it.
The directive is designed to prevent exploitation of workers by ensuring no EU employee in the EU has to work more than an average of 48 hours a week.
But the UK negotiated an opt out from the directive - which 14 other member states are also signed up to.
But that option has been under threat ever since MEPs - including an overwhelming majority of Labour members here - voted to scrap it.
If the issue is not resolved by May, when the current European Parliament ends, the existing working time rules will remain in force.
If the government's arguments are dismissed in these talks, it is feared thousands of part-time or retained fire fighters, who often also have another job, will be forced to give up.
Conservative MEP Philip Bushill-Matthews said: "The Labour government must instruct its MEPs to act in the interests of UK and EU workers.
"Fifteen EU nations now take advantage of the flexibility provided by the opt-out and none of them should back down.
"Scrapping the opt-out during the downturn is even more nonsensical than before and workers should be able to choose if they want to work longer hours to help pay the bills."
UK unions support the MEPs' stand, insisting Britain's culture of long hours must be curbed.
But employers' groups back workers' freedom to choose to work beyond 48 hours if they wish.
Business Secretary Peter Mandelson has also warned that removing the opt-out would be "absurd" and vowed that the government would work to retain it.
After the European Parliament vote in December he commented: "Millions of employees and businesses in the UK and across Europe have benefited from freedom of choice on working hours for many years.
"To take that choice away would be absurd. We're determined to protect the opt-out. Our focus is on helping businesses and employees through these testing times - not making their lives harder."