The heritage building at 57, Whitehall, close to the British Parliament and the Prime Minister's residence, covers 580,000 square feet spread over 7 floors connected by more than three km of corridors.
The Hindujas, Britain's wealthiest people, acquired the building, where war-time prime minister Winston Churchil plotted Second World War manouevres, in partnership with Spanish industrial company Obrascon Huarte Lain Desarrollos (OHLD).
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The building will be restored and redeveloped into a 5-star hotel and apartments. It will also feature a series of private-function rooms as well as a spa and fitness facility.
It was sold on a 250-year lease arrangement following a competitive marketing process.
The Group Chairman S P Hinduja and Co-Chairman G P Hinduja said on behalf of the company, "We will make every effort to honour the heritage and restoration of this national monument, elevate its status and reconnect it with the public."
"We will forge a new future for the site, creating a vibrant and sustainable destination that retains and enhances the historical importance of the building. We are proud to be playing such a crucial role in this new chapter for the Old War Office," they said.
Previously inaccessible to the public, a commitment has also been made to grant new access to the building with an outdoor public space also part of the re-development plans.
Britain's Ministry of Defence confirmed that the Hinduja Group, in partnership with OHLD, who's main shareholder is Grupo Villar Mir, has been successful in the acquisition of the Old War Office.