Builders working on the terminal five site at Heathrow airport look set to go on strike in a dispute over pay.
The GMB union said support for industrial action among its 1,500 members there was “overwhelming”.
The union, which is calling for an increase in bonuses it claims have been frozen for three years, will announce the ballot results later.
GMB union spokesman, Tom Kelly, said: “The feedback that we get from the workers on the site is that their will be overwhelming support for industrial action.”
The terminal five building site – which is as big as Hyde Park – is one of the largest construction sites in Europe.