Former Qatar prime minister Hamad bin Jassim bin Jaber Al Thani is believed to have paid about £150 million for the Grade II listed property and is spending a similar amount converting it into a vast and opulent super-mansion in the heart of Belgravia.

Drawings submitted with a planning application show the finished six-storey house, set back from the road in an acre of land, will have more than 50,000 sq ft of living space, making it one of the largest private homes in London.

The Standard has learned that the sheikh, known as HBJ, bought the Georgian property in 2016 from the Barclay brothers, Sir Frederick and Sir David, owners of the Daily Telegraph. The deal involved the transfer of two shares in a British Virgin Islands registered company to the sheikh.

Pictures show a magnificent double staircase leading up from a cavernous entrance hall flanked by marble pillars. The ground floor will have a huge drawing room and dining room as well as a library and guest salon. The “upstairs, downstairs” layout will include “his and hers” master suites on the first floor, six children’s suites on the upper floors, five guest suites and 12 staff bedrooms —  11 of which will be on the lower ground floor. A spa in the basement will host two plunge pools, a Turkish bath, a gym, sauna, beauty room and treatment room. Previous plans for a large swimming pool have been scrapped.

The basement will also have a cinema and a luggage room, and a second underground level is being dug out to be a garage accessed by car lift. The children’s floors on the second and third levels have their own games and media room and two living rooms. A heritage report submitted to Westminster council with the planning application says the scheme would restore the building to “a single family house of the highest quality”. It adds: “There are few individuals who could/would be able to restore it while ensuring what is truly remarkable about it is both preserved and enhanced.”

Sheikh Hamad, 57, was premier of Qatar from 2007 to 2013, when he spearheaded an investment drive that led to its capital being dubbed “Londoha.” Trophy assets snapped up or funded at the time included Harrods, the Shard, Chelsea Barracks and Canary Wharf. The sheikh also backed the Candy brothers in the One Hyde Park scheme and bought a triplex apartment there. His wealth is estimated at $1.2 billion and his other assets include a 436ft superyacht, Al Mirqab. The Belgravia property has not been occupied as a home since 1941. It was built in 1810 to designs by Sir Robert Smirke, and the interiors were used in an Alfred Hitchcock thriller in the Fifties.The Barclays bought it in 2010 and it has been empty since. The brothers owned it through “offshore entities” in Jersey and the British Virgin Islands as a London base for Sir David’s son Aidan.

Conversion is well under way, with a digger on the land. It is believed work on the grounds and basement will be completed by February. Internal renovations are being carried out by a separate team and could take a further 12 months, sources said. Property experts said a finished value of more than £300 million was “entirely plausible” as the house would be priced at at least £6,000 per square foot if it ever went on the market.

It will be one of only a handful of private central London homes bigger than 50,000 sq ft. Steel tycoon Lakshmi Mittal owns one in Kensington Palace Gardens, known as the Taj Mittal, around the same size. The heritage report, by DIA Historic Buildings Consultancy, said the sheikh’s house would be returned to its status as a “small urban palace” enjoyed by aristocrats when “the British Empire was at its zenith and as rich as it ever was”.

News of the sale emerged as the Barclays, who are based in the Channel Islands, were reportedly searching for buyers for a number of their companies, including the Telegraph titles and the Ritz casino. It follows disappointing financial results from some of their businesses. The brothers have repeatedly denied the Telegraph is for sale.

The sale to the sheikh was certified by a Panamanian law firm, Arias Fabrega & Fabrega Trust Co, BVI Limited. On its website it says it has “a strong offshore practice, for which it has developed … affiliated offices in London, Luxembourg, Geneva, Hong Kong, the British Virgin Islands, Uruguay and Belize”.

Land Registry records for the building show a £34 million transaction. However, it is understood that figure relates only to the lease — the freehold is owned by the Duke of Westminster — with the true cost of the sheikh’s purchase contained within the company transfer.