Specialist Erith Demolition is currently clearing the site with main construction works expected to start early next year.
The £40m contract will deliver 78,100 sq ft of new offices and 37,900 sq ft of retail space directly opposite the Dean Street entrance to the Tottenham Court Road Crossrail station.
Completion is expected by the end of 2021.
Great Portland Estates also confirmed a current pipeline of 11 uncommitted projects totalling 1.3 million sq ft.
It said: “These schemes include a number of exciting projects, including New City Court, SE1 in the London Bridge Quarter, where we hope to materially increase the size of the existing 97,800 sq ft building to more than 370,000 sq ft and will be submitting a planning application early next year.
“At Mount Royal, W1, located at the western end of Oxford Street, we are drawing up early plans to redevelop this two-acre site into a retail-led development scheme.”