Employers say work on the fifth terminal is ahead of schedule

Scaffolders, labourers and other workers have already held a two-day walk out in a row over bonus pay.

The GMB union expects hundreds to join the action on Friday and Monday, and has banned weekend overtime.

A Heathrow spokeswoman said 850 of an 8,000-strong workforce are expected to take part, stressing no delay would be caused to the T5 programme.

Workers, who staged two 24-hour strikes before Christmas, want an extra �1 an hour in their bonus schemes but contractors Laing O'Rourke are offering 22p.

A Heathrow spokeswoman added: "Terminal 5 is in excellent shape and remains on budget, is over 70% complete and slightly ahead of schedule and this action will not cause any delay to the T5 programme."

Terminal 5 is being built to the west of Heathrow Airport and is due to be completed in 2008.