Workers who maintain Royal Mail offices are to take industrial action, including a half-day strike, in a row over the use of tracking devices in their vans.
More than 500 members of the Communication Workers Union employed by Romec will ban overtime and call-outs over the bank holiday weekend and will walk out next Tuesday afternoon.
The union claimed that information from the tracking devices, including clocking-on and off times and travel details, were being used in disciplinary cases, against an agreement reached two years ago with the company, which is jointly owned by Royal Mail Group and Balfour Beatty.
One employee has been sacked and a number of disciplinary cases have been taken, with information from the devices being used as evidence, said the union.
CWU official Ray Ellis said: "Work will pile up because of the overtime ban, while emergency callouts will not be made over the weekend to deal with any problems with fire and burglar alarms or power failures.
"We are asking Romec to abide by the agreement they made with us two years ago."
A Royal Mail spokesman said: "No employees of Royal Mail Group are involved in this action. We are very confident that the robust contingency plans we have in place will ensure there will be no impact on our customers from the planned industrial action."