From 16 January, members of Prospect and the Public and Commercial Services union will launch a ‘work to contract’ for up to 10 weeks. The action will progressively slow down and disrupt the agency’s work on planning and implementing new road schemes, billing and meetings with contractors.

More than 1,500 members of both unions voted for action in a ballot just before Christmas. They were rejecting a pay offer for 2005 which would mean a pay cut for the most experienced staff.

The increase – worth just 0.5% for 220 staff on the maximum of their scales – was imposed by management late last year. For the remaining staff it is worth an average 3.48%.

Prospect negotiator John Higgins said: "The action is designed to persuade management to get back into serious pay talks for 2006. We share many common objectives but there must be provision for a sensible increase for the longest-serving staff.

"It is a measure of the anger felt by members that 85% of those involved are not taking action on their own behalf, but to defend the interests of their colleagues who have been subject to such unwarranted discrimination.

"The action will hit the Highways Agency’s ability to meet its performance indicators and deliver its business plan, which largely depends on staff goodwill because of the amount of travelling involved."

The agency is based at eight centres around the country at Leeds, Manchester, Birmingham, Bedford, Bristol, Dorking, London and Exeter, together with seven traffic regional control centres and their out-stations.

Staff taking action include civil engineers, bridge engineers, environmentalists, traffic management officers and staff employed on finance, procurement and administration.