The 379,000 sq ft scheme at 1 & 2 New Ludgate comprises two distinct buildings united by a new public piazza.
The actual construction contract is worth £109m.
There will be 33,000 sq ft of restaurant and retail facilities at ground and basement levels as well as improved pedestrian links and new public areas.
Demolition work is already complete and construction is scheduled to last until April 2015.
Colette O’Shea, Land Securities Head of Development, London said: “Our decision to commence the speculative development of 1 & 2 New Ludgate reflects our confidence both in the City office market, where we believe supply of new space will be constrained in 2015, and in the quality of the attractive and highly efficient office space we are creating.
“1 & 2 New Ludgate, designed by architects Fletcher Priest and Sauerbruch Hutton, will be a striking, flexible and contemporary addition to one of London’s most historic locations with appeal to a wide range of occupiers seeking space in central London.”