Guidance on how the European working-time directive will affect the NHS was published last week amid a growing row about plans to extend the rules to junior doctors.
The guidance says employers will have to take 'all reasonable steps' to make sure staff do not work more than 48 hours a week over a 17-week period.
And they will have to keep records that 'are adequate to show they have complied with the weekly working-time directives'.
Staff who wish to work longer hours can, but only if they agree it in writing with their employer.
Special arrangements have been made to exempt career-grade doctors and consultants from parts of the directive as it would have affected delivery of care in the NHS.
Junior hospital doctors are currently excluded from the deal.
But European transport commissioner Neil Kinnock and employment and social affairs commissioner Padraig Flynn put forward proposals to extend the directive to lorry drivers and junior doctors last week.
Andrew Hobart, chair of the British Medical Association's junior doctors committee, gave a cautious welcome to plans to extend the directive.
'There is clear evidence that doctors working long hours cannot provide patient care safely,' he said.
'Seven years after the introduction of the New Deal, there are still one in six junior doctors on call for more than 72 hours a week. But the directive has to be introduced sensibly, with some necessary flexibilities.'
NHS Confederation chief executive Stephen Thornton was also concerned. 'This proposal has quite major implications,' he said. 'If we need more consultants, it is a 15-year time scale.'
Mr Thornton also hit out at the NHS Executive for not keeping the NHS informed about European initiatives.
'The NHS is not very European Union aware - and I include the confederation in that,' he said.
'But there is a failure of the NHS Executive to be well informed on EU issues and their impact on the service. We need to know what is going on in Europe.'
A Downing Street spokesperson said it would be 'working with the commission' to make sure that 'when it is finally agreed, the directive reflects the practical needs of the NHS, its patients and staff'.
Clocking on: the working-time directive
Staff should work an average 48-hour week over a 17-week period - unless they sign a written agreement to the contrary.
Staff required to 'sleep in' should include that as working time - so employers may need to review these arrangements.
Where staff work for more than one employer, all employers must take 'reasonable steps' to ensure that they do not exceed working-time limits.
Night staff must not exceed an average of eight hours in 24 over 17 weeks - so 12-hour shifts are still possible.
Staff are entitled to an 11-hour rest between each working day in a 24-hour period.
Working time regulations: implementation in the NHS. www.open.gov.uk/doh/coinh.htm