Brussels will throw down the gauntlet to Britain this week by pushing for tighter rules on how many hours people can work at a stretch.
The European Commission will on Wednesday propose ways of making it tougher for firms to break the 48 hours per week limit laid down by an EU law called the working time directive.
The proposed changes would make it harder for firms to go directly to individual employees to seek their approval for an opt-out, closing loopholes which the Commission has said are abused particularly by firms in Britain, EU sources said.
Business lobby groups and some member states - Britain in particular - have resisted any such change and have champions within the European Union executive Commission.
But Commission insiders said on Friday that would not prevent the proposal being pushed through on Wednesday.
"There is no doubt that it will go through the Commission," said one EU source.
The European Commission declined to comment.
EU sources said the Commission's proposals would include scope to respect national practices.
But Wednesday will just mark the start of the next phase of the battle as London and other opponents of the proposals are expected to try and block a clampdown on working time limits by lobbying EU governments, who must endorse any revamp of the law.
"The UK objectives remain the same... We have support from a range of other member states on both these objectives," a British official said.
The EU executive had hoped to sweeten its proposals by extending the reference period over which the average time worked in a week is calculated to 12 months.
That would compare with the current law which sets the reference period at four months, with the possibility of extending this only by collective bargaining.
But business is unlikely to be satisfied.
"The way they are suggesting setting up the extension of the reference period is complicated and not put in the simple way we wanted," said Therese de Lierkerke, director of social affairs at UNICE, the employers' association.
The Commission can also expect to come under fire from trade unions, who had wanted the opt-out to be phased out and are angered by some of the concessions offered to opponents of such change.
The European Parliament, which also has a say on any changes to the working time law, might also quibble with what trade unions will argue is a watered down version of what they wanted.
While the battleground will move to meetings of EU employment ministers, the war could drag on longer - an EU source said a draft of the proposal included mention of a review of changes after five years.
Still, there is a consensus on the need to change some elements of the law on working time, particularly the way that working time is defined.
This issue came to a head after a court ruling last year forced Germany and others to count all time that doctors spent in hospitals as standard working time, regardless of whether they were asleep while on call.
The Commission's proposal would seek to count only a proportion of the time spent on call as working time to take account of the time likely to be spent resting.