Princess Beatrice and husband Edoardo Mapelli Mozzi attend opening of new luxury hotel The OWO

The reimagined hotel features 120 rooms and suites starting at £1,100 per night

Princess Beatrice was among a host of royals and celebrities who gathered for the re-opening of London’s historic Old War Office (OWO) on Tuesday, welcoming its new era as a super-luxe hotel.

Princess Anne, Lord Andrew Lloyd Webber and Italian tenor Andrea Bocelli were also among those attending the Whitehall landmark’s glamorous curtain raiser, which first opened in 1906 and was later used by Winston Churchill during the Second World War.

Beatrice and Mapelli were all smiles as they attended the event last night - posing for photographs and mingling with stars of TV and stage.

Their evening out came after the couple celebrated their daughter Sienna’s second birthday.

Princess Beatrice of York and husband Edoardo Mapelli Mozzi attend the inauguration of The OWO hotel (Dave Benett/Getty Images for Raf)

Princess Anne shakes hands with Gopichand P Hinduja, Chairperson of the Hinduja Group, at the inauguration (Dave Benett/Getty Images for Raf)

Indian billionaires, the Hinduja brothers, purchased the 580,000 sq ft Whitehall Palace in March 2016 for in excess of £350 million.

Princess Anne met with the development group behind the monumental six year renovation before unveiling a plaque and officially inaugurating its new life as a super-luxe hotel and residency.

Built on the historic royal site of Whitehall Palace, Britain’s seven-storey Old War Office was bombed eight times in the Blitz, has 1,000 rooms, and two-and-a-half miles of sprawling corridors.

The Edwardian monument offers 89 guest rooms with even the shortest stay starting at over £1,000 per night, while longers breaks in one of the 21 suites could quickly exceed tens of thousands of pounds.

While a renowned Argentinian chef will oversee a three Michelin star menu which VIPs will be able to enjoy around the clock, before working off their meal in the state-of-the-art gym and spa.

The OWO’s remarkable history

With construction beginning in 1899, by the time the building was completed seven years later, it had used 26,000 tonnes of Portland stone, 3,000 tonnes of York stone and 25 million bricks. The full cost, revealed in parliament in 1910, put the cost of the new War Office at £1,229,128.

During the building’s early years, Mr Asquith, Lord Kitchener and Winston Churchill occupied its offices as Secretaries of State for War, with the War Office remaining at the centre of the British Army’s administrative work until the unified Ministry of Defence formed in 1964. As such, much of the department was transferred.

Notably, Ian Fleming, the mastermind behind the James Bond franchise, was a regular visitor to the building during its heyday and multiple scenes from the films have been filmed at the Old War Office.

While Whitehall Palace was later described by T. E. Lawrence , who in 1914 was employed in the Geographical Section of the War Office, as being of such opulence that it was ‘only for the use of Field Marshals and chairwomen’.

The building has never been open to the public, but the multi-million-dollar investment has seen it transformed into the UK’s first Raffles hotel.

The reimagined rooms start at approximately £1,100 per night, featuring 120 rooms and suites, a 600-person ballroom and London’s first Guerlain-branded spa.

The renovation took eight years, with the building’s historic elements, including its original oak panelling and hand-placed mosaic flooring, restored by hundreds of experts.

Lord Andrew Lloyd Webber and Lady Madeleine Lloyd Webber at the hotel’s opening on Tuesday (Dave Benett/Getty Images for Raf)

Italian tenor Andrea Bocelli performs on the evening of the hotel’s opening (Dave Benett/Getty Images for Raf)

“We needed the right spot,” Philippe Leboeuf, managing director of the hotel, told CNN Travel. “And we felt that this was the building for it.”

“When we came to Whitehall, the team were blown away by the size and beauty of this majestic building,” Sanjay Hinduja, who has overseen the project, told The Hindu Business Line.

“No expense has been spared in bringing it back to its former glory and paying homage to its heritage, whilst breathing new life into it. Along with Raffles London at The OWO, we hope to create a legacy that is both timeless and unsurpassed.”