The Enquirer understands that the deal to fit-out 5 Broadgate is worth around £100m.
Banking giant UBS will occupy the 111,000 sq metre scheme which is currently under construction with Mace as project manager.
Work started in April 2012 for developers British Land and Blackstone with UBS set to move in by the end of 2014.
UBS is now believed to be talking to some of the biggest fit-out names in the business about the mega contract to make the building ready for 6,000 banking staff who will occupy the new HQ.
The Enquirer understands that names in the frame include ISG, Overbury and Mace itself.
One industry source said: “This is one of the biggest fit out deals to hit the market for a while so the bidding race is sure to be ultra-competitive.”