Princess Anne, 73, dressed in a blue satin polka dot skirt suit officially opened The OWO – Whitehall's former Old War Office – following a six-year renovation.
Before officially unveiling a plaque to commemorate the inauguration, Anne met with representatives from the Hinduja Group, who have spearheaded the project.
Meanwhile, Princess Beatrice, 35, and property developer Edoardo, 39, enjoyed a date night at the ceremony, and posed for photographs on the opulent marble staircase.
Beatrice looked elegant in a floral Erdem dress with a black blazer and matching heels, while her husband of three years, donned a blue suit.
The couple celebrated their daughter Sienna's second birthday last week, with Edoardo sharing a rare photograph of the tot.
Guests at The OWO's inauguration were treated to special live performances by Italian tenor Andrea Bocelli and composer Lord Andrew Lloyd Webber.
The architectural landmark opened to the public for the first time in the summer and features the UK's first Raffles Hotel, in collaboration with New York-based designer, Thierry Despont.
It also includes 85 private residences, nine restaurants, three spars, Guerlain's first London spa and active wellness by Pillar Wellbeing.
Earlier today, the Princess Royal carried out an investiture at Windsor Castle, including presenting the Queen's Gallantry Medal to Lukasz Koczocik, who helped tackle the Fishmongers’ Hall terror attacker.
Mr Koczocik, 43, was one of four men who confronted convicted terrorist Usman Khan during the attack in November 2019.
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