The contractor will work across ten floors to refurbish 80,000 sq ft of office accommodation.
The contract at Basinghall Street also involves upgrading the façade of the 21-storey building which was built in 1964 and last refurbished in 1985.
ISG has won a £16m project to refurbish and remodel City Tower in the City of London for Great Portland Estates and Starwood Capital.
The contractor will work across ten floors to refurbish 80,000 sq ft of office accommodation.
The contract at Basinghall Street also involves upgrading the façade of the 21-storey building which was built in 1964 and last refurbished in 1985.