The six-storey building at 148 Old Street is currently occupied by the Post Office.
Great Portland now owns the site via its Great Ropemaker Partnership joint venture with the BP Pension Fund.
Royal Mail Group currently pays a rent of £2.17m a year per with a rent review and tenant break option in May 2015.
Great Portalnd said: “The property sits on a site of 0.83 acres and provides an exciting opportunity for refurbishment or redevelopment from 2015.”
The Old Street area has been dubbed “Silicon Roundabout” and is at the heart of London’s expanding hi-tech industry.
Toby Courtauld, GPE Chief Executive said “This is a well-located site providing opportunities to create significant value over the medium to longer term in this rapidly changing part of central London.”