BRITAIN’S biggest-ever theft has just come to light. An audit and valuation of the works of art stolen in February from the Wiltshire manor house of Harry Hyams, a retired property developer, has put their value at £80m.

The scale of the theft easily exceeds the £53m in cash taken from the Securitas depot at Tonbridge, Kent, in the same month.

The £80m figure, reported on The Art Newspaper’s website this weekend, is an estimate for insurance purposes, but it is believed that the market value of the items stolen is only slightly lower. It was initially thought that the value of 300 pieces taken was £30m.

It has emerged that Hyams has over the past 50 years quietly transformed the 17th-century Ramsbury Manor, near Marlborough, into one of Britain’s greatest private art collections. It is surpassed only by that of the Queen and by a handful of ducal palaces and stately homes whose collections have been built over centuries.

The extent of Hyams’s losses and of what he owns has only emerged now that police and art experts have completed a full assessment. Initially it was thought that Hyams had lost the bulk of his collection in the raid. Friends, however, have revealed that the theft involved only “a small fraction” of the works he owns.

“The theft, while very upsetting, is only a dent in his entire collection,” said Dorrit Moussaieff, the jewellery designer who is an old friend of Hyams. She added: “Harry is a buyer and everything he buys is 10 out of 10 in quality. His art and his collection are only the very, very best.”

Other art experts who know Ramsbury agreed with her assessment.

Hyams, 78, has told friends he intends to leave the house and its works, mainly from the 17th and 18th centuries, to the nation when he and his wife Kay are dead. The couple have no children. It is understood he has asked his lawyers to begin discussions with the government but has made clear, however, that he does not want the National Trust to take it over.

During the burglary, the thieves broke through a perimeter fence in two 4x4 vehicles. They mainly took smaller objects rather than, for example, the 17th-century carvings by Grinling Gibbons which adorn part of the house.

Nor did they touch the paintings by Turner, Gainsborough, Goya, Rubens and Rembrandt. This may have been because they had limited carrying capacity.

The burglars’ haul did include 17th and 18th-century clocks by Thomas Tompion and his apprentice Daniel Delander and by Benjamin Vulliamy, clockmaker to George III. Other losses include a pair of porcelain busts from the Bow factory in Essex, Vincennes chinoiserie porcelain from France and a highly valuable figure of a poodle.

Hyams made his fortune in building office blocks, most famously the Centre Point building in central London. He earned the opprobrium of much of the Labour party when he left the building empty in the 1970s at a time of housing shortages.

In addition to building modernist towers, Hyams began collecting mainly silver and porcelain in the 1950s before widening his interests to furniture, paintings and clocks.

He is also keen on cars and is believed to own several Ferraris.

Hyams, whose family originally came from Russia, is patriotically British, reflected in his choice of antiques.

“Essentially what he has done is buy objects and works of art for Ramsbury to suit the building as it might have been in the late 17th and 18th centuries,” said Chapman Pincher, the journalist, a neighbour and old friend of Hyams who has also visited him on board his yacht, Shemara.

Pincher and other friends also said that Hyams, who bought Ramsbury for £650,000 in 1964 — then a record for a private property — never disposes of any of his acquisitions.

Some of the items stolen from Ramsbury have been recovered. In March Wiltshire police discovered about 140 of them on waste ground by a garden centre near Stratford-upon-Avon.

The Art Loss Register, which works closely with the police, has put a list of the stolen items on its website along with photographs to help their recovery.

Julian Radcliffe, its director, said: “I suspect some will go overseas if they are not found, but I also reckon that the thieves will delay selling in the hope that things will quieten down and they can later sell them more easily.”

The Ramsbury burglary is one of several recent big art thefts. In 2003 a Leonardo da Vinci painting, The Madonna of the Yarnwinder, valued at £30m, was stolen from the home of the Duke of Buccleuch, and Lord Rothschild had gold boxes stolen from Waddesdon Manor, his Buckinghamshire home.

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