Terminal 5 is the size of Hyde Park

The GMB union is to ballot 1,500 builders on the Terminal Five site at Heathrow Airport on industrial action.

The union says feelings are "running high" about bonus pay which, it claims, has been frozen for three years.

Work on T5, which is the size of Hyde Park, began in 2002 and is due to open in 2008, catering for an extra 30m passengers a year.

GMB national officer Phil Davies said: "GMB and other unions are preparing a ballot for industrial action at Heathrow Terminal 5 following the failure of talks to resolve a pay issue on the site.

"Bonus pay has been frozen for nearly three years and the members want a review of the scheme to regain the lost value.

"Unless there is movement, industrial action is likely as feelings are running high on the issue."

Construction of Terminal 5 began after a record public inquiry following opposition from local people and environmentalists.