Ford and Vauxhall were yesterday targeted for a campaign to cut working time throughout manufacturing industry.
Senior officials of the Amalgamated Engineering and Electrical Union said they would refuse to recommend any pay deal at the two car companies this year unless it included a reduction in hours.
Vauxhall has already offered a 3 per cent rise without cutting hours and Ford unions next week are to claim 10 per cent, together with a reduced working week.
Meeting in Birmingham, members of the union's national industrial committee registered their belief that unless there was a breakthrough on working time at such high-profile companies as Ford and Vauxhall, further progress towards a 35-hour week would be impossible.
A previous drive to reduce working hours - which ended five years ago because of the recession - was generally regarded by unions as one of the most successful campaigns of industrial action since the Tories came to power in 1979.
While yesterday's announcement applied specifically to workers in the motor industry, the AEEU believes it has far wider significance. The Confederation of Shipbuilding and Engineering Unions has accumulated an pounds 11m fund for the campaign which may include the threat of industrial action.
While Rover operates a 37-hour week, Ford and Vauxhall expect employees to work 39 hours.