A jewel thief was jailed for more than five years in Britain on Wednesday for stealing millions of pounds worth of diamonds by switching them with pebbles in a daring sleight of hand.

Lulu Lakatos, 60, posed as a gem expert under the alias "Anna" to gain access to the jewels worth a total of £4.2 million ($5.8 million, 5.0 million euros).

She claimed she had been sent to appraise the gems by a wealthy Russian client before swapping them for the worthless stones.

A jury at Southwark Crown Court in south London found her guilty of conspiracy to steal. The judge called the heist "highly sophisticated and audacious", involving "brazen deception".

Prosecutors said the theft of the seven diamonds -- including one worth £2.2 million -- from a luxury family jewellers in London's upmarket Mayfair district was the biggest heist of its kind ever committed in Britain.

The real diamonds, which London police have said were stolen by Lakatos in cooperation with an international organised gang, have never been recovered in the wake of the March 2016 theft.

Romanian-born Lakatos, from the Saint-Brieuc region of northwest France, arrived in London a day before the theft where she met with two other members of the gang.

The trio then made a reconnaissance trip to Boodles' jewellers on Bond Street in Mayfair.

On the day of the theft, the court heard Lakatos had examined and weighed the diamonds, wrapping them in tissue paper and placing them in boxes in a locked purse.

Security camera footage from the family firm's basement showed the moment Lakatos switched the purse for a duplicate in her handbag using what the court heard was "sleight of hand".

The practised thief left the jewellers and switched the diamonds into the handbag of an unknown woman.

She then changed her clothes in a pub toilet and made her escape for France on the Eurostar cross-Channel train service, less than three hours after committing the crime.

The conviction follows Lakatos' arrest in France on a European arrest warrant.

She had three previous convictions for theft in France and was wanted in Switzerland for a similar heist where an envelope containing 400,000 euros was switched for a copy holding worthless paper.

In her defence at trial in London, Lakatos claimed she had been arrested in a case of mistaken identity and her younger sister -- who died in a car crash in October 2018 -- had used a passport in her name to travel to Britain to commit the crime.

The two men who accompanied Lakatos, Christophe Stankovic and Mickael Jovanovic, were jailed for three years and eight months after pleading guilty to conspiracy to steal.

A jewel thief was jailed for more than five years in Britain on Wednesday for stealing millions of pounds worth of diamonds by switching them with pebbles in a daring sleight of hand.

Lulu Lakatos, 60, posed as a gem expert under the alias "Anna" to gain access to the jewels worth a total of £4.2 million ($5.8 million, 5.0 million euros).

She claimed she had been sent to appraise the gems by a wealthy Russian client before swapping them for the worthless stones.

A jury at Southwark Crown Court in south London found her guilty of conspiracy to steal. The judge called the heist "highly sophisticated and audacious", involving "brazen deception".

Prosecutors said the theft of the seven diamonds -- including one worth £2.2 million -- from a luxury family jewellers in London's upmarket Mayfair district was the biggest heist of its kind ever committed in Britain.

The real diamonds, which London police have said were stolen by Lakatos in cooperation with an international organised gang, have never been recovered in the wake of the March 2016 theft.

Romanian-born Lakatos, from the Saint-Brieuc region of northwest France, arrived in London a day before the theft where she met with two other members of the gang.

The trio then made a reconnaissance trip to Boodles' jewellers on Bond Street in Mayfair.

On the day of the theft, the court heard Lakatos had examined and weighed the diamonds, wrapping them in tissue paper and placing them in boxes in a locked purse.

Security camera footage from the family firm's basement showed the moment Lakatos switched the purse for a duplicate in her handbag using what the court heard was "sleight of hand".

The practised thief left the jewellers and switched the diamonds into the handbag of an unknown woman.

She then changed her clothes in a pub toilet and made her escape for France on the Eurostar cross-Channel train service, less than three hours after committing the crime.

The conviction follows Lakatos' arrest in France on a European arrest warrant.

She had three previous convictions for theft in France and was wanted in Switzerland for a similar heist where an envelope containing 400,000 euros was switched for a copy holding worthless paper.

In her defence at trial in London, Lakatos claimed she had been arrested in a case of mistaken identity and her younger sister -- who died in a car crash in October 2018 -- had used a passport in her name to travel to Britain to commit the crime.

The two men who accompanied Lakatos, Christophe Stankovic and Mickael Jovanovic, were jailed for three years and eight months after pleading guilty to conspiracy to steal.