The 2.3 acre Rathbone Place site was formerly occupied as a sorting office by the Royal Mail Group.
Sources close to the job confirmed that Lend Lease has been awarded the pre-construction contract.
The main contract is believed to be worth at least £150m.
The source said: “It obviously puts Lend Lease in a strong position for the main deal but nothing is definite at this stage.”
Construction information specialist Glenigan listed Lend Lease, Brookfield Multiplex, Sir Robert McAlpine and Mace as bidders for the main deal.
Great Portland Estates paid £120m for the site in 2011 and leased it back to the Royal Mail.
Architects MAKE have drawn-up designs to provide new buildings totalling 414,000 sq ft, which will include a new public square and new pedestrian routes linking Rathbone Place and Newman Street together with 42,000 sq ft of retail space, 217,000 sq ft of offices and 155,000 sq ft of residential in 162 flats.
The scheme is believed to have a development value of around £500m.