Heathrow's problems have mounted in the past year

GMB members working for Emcor, which provides maintenance, cleaning and security services at the airports, will walk off the job on 7 September.

The union said 100 workers would strike after members overwhelmingly rejected the firm's pay offer in a ballot.

But Emcor insisted only about 50 staff were likely to be involved, and there would be no disruption to passengers.

'No difference'

Should the stoppage go ahead, Emcor said the striking workers would be replaced by temporary staff and managers to ensure there was no disruption.

"Passengers won't even notice or see any difference whatsoever," a spokesman said.

But GMB said this raised "grave" security concerns since the striking workers were responsible for vital services such as maintaining and repairing aircraft tow trucks and passenger ramps.

Emcor can afford to pay a fair increase to our members who live in the expensive south east Rob Kelsall, GMB

It said 81% of participating members had voted for the strike, the first ever at Emcor.

GMB said staff were being offered a 3% annual pay rise, some way below the level of retail price inflation.

"They are very angry and have deemed the offer as a kick in the teeth," said GMB official Rob Kelsall, adding that workers had to a right to share in the firm's growth.

"Emcor is a profitable company and it can afford to pay a fair increase to our members who live in the expensive South East."

Emcor maintains facilities at passenger terminals, aircraft hangars and cargo centres on behalf of British Airways.

It said its pay offer was a fair one and that two thirds of its 320 staff at Heathrow and Gatwick had accepted it.

Mounting problems

Holidaymakers and businesses have been counting the cost of mounting delays at Heathrow over the past year.

The legacy of 2006's terrorist bomb alert, which led to a tightening of security procedures at British airports, and staffing problems at Heathrow have piled pressure on BAA, the airport's operator.

Ministers and business groups have warned that the UK's economic competitiveness could be seriously damaged if capacity problems at Heathrow are not addressed.