Hundreds of workers are expected to strike

Hundreds of scaffolders, labourers and others manned picket lines on Friday, the start of the four day stoppage.

There will be an overtime ban over the weekend and another strike on Monday as part of campaign for a �1-an-hour rise in bonus pay.

Employers Laing O'Rourke said their offer of an extra 67p-an-hour would be withdrawn if the strike went ahead.

But the GMB union said the offer had not been received in writing.

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

"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," she said.

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