Business leaders have pleaded for employees to be allowed to work more than 48 hours in a week.
Men in the UK work an average 40 hour week
The CBI fears that removing employees' right to opt out of the European Working Time Directive could be catastrophic for business.
They say new research involving 400 firms reveals the that a "significant or severe" impact on business would be likely.
Director general Digby Jones said the opt-out, which the European Commission will debate later this year, would show where the government really stood on the need for a flexible labour market.
"People should have a right to say no to long hours and the directive rightly
gives them that protection.
"But they don't want unions and politicians telling them when they can work or for how long.
"That would be over-zealous interference of the nanny state. Further
restrictions on working time would be a kick in the teeth for many firms,
particularly smaller ones."
The government must act to stop exploitative bosses from squeezing even more out of their overworked staff
But Mr Jones stands in direct opposition to the unions, with TUC general secretary Brendan Barber saying UK businesses are "obsessed"
with making workers accept long hours.
He said: "The government's own statistics suggest that 2.5 million employees want to spend less time at the office, even if fewer hours mean less money.
"Many workers simply don't get a choice whether or not to work long hours - some feel bullied into staying late, for others the workplace culture means that leaving on time is seen as letting the team down.
"Working long hours can lead to unnecessary stress and people with excessive days are more likely to have accidents at work.
"Most European countries have set
their working time limits below 48 hours, and the UK is the only EU country still with an opt-out.
"The government must stand firm against this employer onslaught and act to stop exploitative bosses from squeezing even more out of their overworked staff by refusing to renegotiate another opt-out when the current one expires later this year."
Meanwhile, merged union Amicus has attacked the government for refusing to apply the directive to offshore workers, which would entitle them to four weeks paid holiday.
Amicus said ministers were fudging over the issue and accused oil companies pleading the unaffordability of giving their workers paid holiday.
Derek Simpson, joint general secretary, said: "We are demanding that the
government ensures the Working Time Directive is applied in full and in
particular offshore workers are awarded four weeks paid holiday."