British employees work the longest hours in Europe with 16 percent of workers, four million people, breaking the European Union's 48-hour weekly limit, according to a Commission report.
The largest British employers' organisation, the Confederation of British Industry (CBI), says 33 percent of the workforce sign an opt-out agreement with companies, waiving the right to a 48-hour week.
"We can't be sure that workers who sign an opt-out agreement at the same time as an employment contract have a real choice," EU Employment and Social Affairs Commissioner Anna Diamantopoulou told a news conference.
The Commission is reviewing the EU working time directive which says employees cannot be forced to work more than 48 hours a week, on health and safety grounds.
Britain has made the most extensive use of the opt-out, sparking concern among trade unions that employees' rights are being ignored.
"Many (workers) often feel pressurised by their employers to sign themselves out of working time protection," the Trades Union Congress (TUC) said in a statement, welcoming the Commission's report.
"Removing the individual opt-out would help signal the end of Britain's unhealthy long-hours culture." Diamantopoulou said the 48-hour working limit would not be raised in the new proposal.
However, she called for flexibility in the health sector to curb the long hours worked by doctors without causing an explosion in costs, a key concern for Germany, the Netherlands and Britain.
The Commission would come forward with draft legislation to revise the existing rules by the autumn at the earliest.