The Terminal Five site is the size of Hyde Park

The GMB union wants an increase in bonuses which, it claims, have been frozen for three years for those working at Heathrow's Terminal Five.

Employers Laing O'Rourke claimed turnout was "significantly below the 50% needed to legitimise the vote".

But the union denied this and said 98.6% of members balloted had backed industrial action.

Tom Kelly of the GMB said the ballot's unanimous result showed the "strength of feeling" among members.

'Highest paid'

He said: "The builders and civil workers have made this site what it is.

"They're the ones who have cleared it, they have built it and got it on time and within budget."

But a spokesman for the construction firm said: "During today's ballot at T5 only a minority of Laing O'Rourke's workforce actually voted.

"Workers at T5 are some of the most productive and highest paid in the industry and T5 has been a very progressive project in all respects.

"We continue to hope to resolve this issue amicably and will be seeking further discussions with the unions."

A spokeswoman for airport operator BAA said if there was a strike, contingency plans would be implemented.

She said there were 528 union members among 930 workers at the site who worked for Laing O'Rourke.

The building site is as big as Hyde Park and due to be finished by 2011.