Videos posted on social media show torrents of water flowing through the site in King’s Cross.
ISG is undertaking fit-out work on the 415,000 sq ft office complex where main construction was carried-out by BAM.
One video shows a site worker stripped down to his shorts in a mobile elevated work platform battling to stop the flow of water.
Another shows water cascading down through the lower floors of the 12-storey building.
ISG declined to comment.