James Stunt, the former husband of Formula One heiress, Petra Ecclestone, has admitted not owning a front door key for his £10 million Belgravia home, as he described being the victim of one of the largest thefts in UK history.

The 38-year-old gold bullion dealer claimed diamonds given to him by a "deceased member of the royal family" were among £30 million worth of gems that went missing from the safe of his London property.

His former security guard, Justinas Ivaskevicius, 34, is on trial at Southwark Crown Court accused of stealing a 16.18-carat fancy yellow diamond ring valued at £515,000.

But Mr Stunt said other more valuable items of jewellery had also been taken when the safe was completely cleared out, but for a single symbolic screw.

He explained: "I lost diamonds, some gifted to me. I’m not going to say from who in this court because it was a deceased member of the royal family."

He said he had never had a key for the safe himself and did not even have a front door key to his own home.

Mr Stunt told the jury: "Believe it or not, I didn’t even have a key to enter my own house.

"I was always let in because someone was always present. I thought, ‘if there’s only one key, how can it be a problem?’ That was the security measures that I thought were necessary."

Mr Stunt said the safe had been completely emptied. "The whole thing had been cleaned out. There was literally not a receipt left. They took both my passports – my diplomatic and citizen ones.

As a picture of a safe containing only a single screw was shown in court, Mr Stunt joked: "Maybe that was a signal, ‘you got screwed’."

Mr Stunt described himself as an "extremely wealthy" man at the time of the alleged theft, who had a gold assaying business as well as being an investor and dealing in fine art.

Mr Stunt had bought the gem as a diamond ring for £374,000 from Bond Street jewellers Mossaieff on November 30 2017, the court heard.

Prosecutors claim Mr Ivaskevicius, posing as "Sebastian Thomasz Kowal", tried to sell the diamond to a jewellery dealer in Antwerp, Belgium before he was caught.

The jury heard that the police investigation into the diamond theft also revealed that Mr Ivaskevicius was involved in a cannabis farm, the possession of criminal cash, handling stolen cheques and the use of a number of false identities.

Mr Ivaskevicius denies theft, two counts of possessing criminal property, possessing an identity document with improper intention, producing a class B drug and four counts of handling stolen goods.

The trial continues.