The Enquirer understands that O’Rourke has been told privately it has won the job ahead of an official contract announcement by client British Land and Oxford Properties.
O’Rourke was in a straight head-to-head with Skanska for the contract.
Sources close to the negotiations said O’Rourke won out because it proposed a shorter construction programme.
The source told the Enquirer: “It was a hard fought contest but O’Rourke was chosen because it came up with a shorter timetable for the job.
“It’s a great one to win and will put O’Rourke firmly back on the map in the London commercial tower market.”
Groundworks have already started on the cleared site and construction work is scheduled to last until late 2014 at least.