Hunt for 'lone wolf' thief who stole jewellery worth $21 million from London mansion

It took one thief just 19 minutes to make off with $21 million in jewellery and designer items in one of Britain's biggest home burglaries.

Detectives from London's Metropolitan Police have released photographs of the valuable, and in many cases, bespoke items stolen from the opulent home on Avenue Road, St John's Wood on December 7.

The street, known as Billionaire's Row, is one of the poshest addresses in London and home to scores of largely international magnates. 

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The burglar entered the 13-bedroom mansion through a second-storey window at 5.11 pm and rummaged through a few rooms before going downstairs and breaking into a safe, The Guardian reports.

He escaped out the same second-storey window at 5.30pm.

About $300,000 worth of Hermès Crocodile Kelly handbags, $30,000 in cash, and jewellery worth $21 million were stolen.

The stolen jewellery included a Graff 10.73-carat diamond ring, two butterfly diamond rings by De Beers, a Hermès 3.03-carat ring, aquamarine ring and "Niloticus Lumière" necklace.

About eight people, including staff and family members, were at home in the five-storey mansion when the robbery took place.

Detective Constable Paulo Roberts said many of the pieces stolen were unique and therefore easily identifiable.

"This is a brazen offence, where the suspect has entered the property while armed with an unknown weapon and violated the sanctuary of the victims' home," he said.

"We urge anyone who was in the area of Avenue Road, NW8, and saw anything suspicious to please come forward. Also, if you have seen this jewellery since, someone has offered to sell you it, or you have any further information, then please also contact the police or Crimestoppers anonymously."

Csaba Virag, the chief of staff to the family and who was in the mansion at the time, told The Guardian video footage showed the thief "had definitely done it before".

The break-in looked like it was carried out by a "lone wolf", but the thief could turn to a wider network to offload the stolen items, Virag said.

The family have offered a reward of up to $1 million for information leading to the capture and conviction of the suspect. An additional reward of 10 per cent of the value of any items recovered is also on offer.