The contract includes estate management and construction services at 106 locations in London and South East England.
It encompasses over 7,000 buildings and facilities including the famous Chelsea Barracks, Horse Guards Parade and Buckingham Palace.
Interserve was awarded the £380m South East regional prime contract back in March 2005, due then to run for seven years.
The extension augers well for the next generation of contracts awards in 2014.