As tipped by the Enquirer in December, the firm has secured the £125m project to deliver the new UK headquarters for UBS at 5 Broadgate ahead of rival fitout specialist Mace/Como.
The new office building is currently under construction with Mace acting as construction manager.
ISG is scheduled to start work on site at the start of 2015, following a 12-month pre-construction period.
Banking giant UBS will occupy the 1.2m sq ft scheme, which is currently under construction with Mace as project manager.
Appointed under a construction management form of contract, ISG will deliver 700,000 sq ft of high-specification commercial office accommodation for the global financial services firm.
The building will house up to three trading floors, enabling UBS to consolidate its London trading operations within one building, an auditorium, large executive area and a range of open plan and cellular office space.
David Lawther, ISG chief executive, said: “Following such an exceptional year’s performance for our London fit-out business in 2013 with a win of this scale and prestige is a tremendous result and further reinforces our position as the sector’s lead contractor.
“Reassuringly, confidence is returning to the London office market as a whole, and in addition to securing many of the capital’s most high-profile schemes, we have maintained and built on our reputation for delivering exceptional fit out solutions across all value bands.”