No strike dates have been set among the 8,000 GMB members and the company, part of Centrica Plc, has been given a week to respond before the union decides on its next move.

"British Gas has turned from being a reasonable employer where people had job satisfaction into one with a culture of bullying, customer exploitation and profits at all costs," Paul Kenny, GMB General Secretary, said on the union's website earlier this month. Matthew Bateman, director of heating services at British Gas, Britain's biggest household energy supplier, said "robust contingency plans" were in place to help maintain customer services.

"We remain committed to talking with the GMB and avoiding this unnecessary industrial action," he added, in a statement.

A strike would be further embarrassment for Labour, already facing jibes from the Conservatives over British Airways cabin crew and planned action by National Rail signal workers, just a few weeks ahead of a national election.

In addition, thousands of civil servants plan a one-day stoppage Wednesday, coinciding with the Budget.

(Writing by Avril Ormsby; Editing by Steve Addison)