Michael Beloff QC, representing the Government in its first legal battle to halt the imposition of social policy by Brussels, said the directive agreed more than two years ago had been brought in without any evidence that the measures "were best advanced at European and not national level".
The European Court of Justice in Luxembourg was asked to find the law was illegal - because the European Commission had pushed it through on "health and safety" grounds when it was really a separate matter of social policy.
The legal distinction is crucial. The Government was unable to veto what it saw as interference from Brussels in domestic social policy because any European laws introduced under the health and safety heading only require majority backing from ministers. Social policy issues require unanimity.
If the court finds the law was legally introduced, Britain will remain subject to a 48-hour maximum working week as the norm, coupled with other restrictions compelling workers to rest for 11 consecutive hours a day; to take a break if the working day exceeds six hours; to have at least one day off a week; and to take four weeks paid holiday a year.
An interim "opinion" will be delivered on 12 March.