About 900 workers walked out, according to the GMB

Picket lines were set up from dawn with GMB union claiming 900 had walked out. A second 24-hour walk-out is planned for Monday and weekend overtime banned.

Workers want an extra �1 an hour in their bonus schemes but contractors Laing O'Rourke are offering 22p.

Laing O'Rourke said they are disappointed, describing T5 workers as some of highest paid in the industry.

The company employs 850 of the 1,500 civil engineering workers on the site. Amec who, together with Balfour Beatty, employ the rest, also made a claim.

The vast majority... have continued working normally today - this action will not cause any delay to the T5 programme BAA spokesman

Both have been in talks with unions, with a further meeting planned for Tuesday.

The last review of bonus pay rates took place three years ago.

There are further threats of industrial action on 20 and 23 January.

Terminal 5 (T5) is being built to the west of Heathrow Airport and is due to be completed in 2008.

A spokesman for airport operator BAA said only a small proportion of the 6,000-strong workforce are involved in the action.

"The vast majority... have continued working normally today. This action will not cause any delay to the T5 programme."