A woman who stole diamonds worth £4.2 million by swapping them for pebbles in the biggest ever theft of its kind has been jailed for more than five years.

Lulu Lakatos, 60, posed as gem expert “Anna”, to swipe the seven stones from luxury Mayfair jewellers Boodles in a plot likened to a Hollywood heist movie.

She was sent to value the diamonds, which were to be placed in a padlocked purse and held in the New Bond Street store’s vault until funds were transferred, on behalf of supposed wealthy Russian buyers.

But CCTV footage from the family firm’s basement showroom captured the moment the purse was put into Lakatos’s handbag and switched for a duplicate in seconds using “sleight of hand”.

Boodles boss Nicholas Wainwright had briefly left to talk on the phone to apparent Russian buyer “Alexander”, who he had met over lunch at Monaco’s Hotel Metropole.

The jewellers’ diamond expert, Emma Barton, raised suspicions but the diamonds, believed to have been concealed in a hidden compartment, were not found in Lakatos’s handbag during a search.

She left the shop before switching the gems to the handbag of an unknown woman and the international gang of criminals fled the UK for France in less than three hours.

When the purse in Boodles’ safe was opened the following day, inside were seven small garden pebbles, and the real diamonds have never been recovered.

Prosecutor Oliver Mosley said: “This was a conspiracy of the highest sophistication.

“It is believed to be the highest value individual theft offence of this kind ever committed in this country.”

Lakatos claimed Anna was her younger sister, Liliana Lakatos, who had confessed to using Lulu Lakatos’s passport to commit the crime months before she died in a car crash, aged 49, in Romania in October 2019.

Liliana Lakatos was wanted in Switzerland for an almost identical plot, where an envelope containing 400,000 euros was switched for a duplicate filled with paper.

But Lulu Lakatos sobbed in the dock after she was found guilty of conspiracy to steal on or before March 10 2016 by a jury at Southwark Crown Court by a majority of 10 to one after nine hours and 19 minutes of deliberations.

Judge Emma Goodall QC sentenced Lakatos to five-and-a-half years imprisonment on Wednesday. “You have been convicted of conspiring to steal diamonds, the jury having rejected your defence it was your deceased sister and not you who perpetrated the offence.

“This was a highly sophisticated and audacious offence in terms of planning, risk and reward, an offence in respect of which you played an integral role,” the judge told Lakatos.

“You spent almost an hour in the Boodles London showroom exploiting their trust, which allowed you to engage in a brazen deception and through sleight of hand you stole seven diamonds valued at £4.2 million.”

Flying Squad Acting Detective Sergeant William Man said the investigation is still ongoing to identify others involved in the heist.

He said: “This was an audacious theft, carried out in plain view of experienced and professional staff at a renowned jewellers.

“The meticulous planning and execution of this theft reveals to me that those involved were highly skilled criminals.”

Senior Crown Prosecution Service prosecutor Thomas Short added: “Lulu Lakatos’ actions were criminal.

“She played an integral part in an audacious swoop that could be likened to a scene from a Hollywood film.”

Romanian-born Lakatos, from Saint-Brieuc, Brittany, has three previous convictions for theft in France.

She arrived in London the day before the theft and was captured on CCTV with Georgeta Danila, 53, entering the Cricklewood Lodge Hotel in north London before making a reconnaissance trip to Boodles with Christophe Stankovic and Mickael Jovanovic.

Danila waited in the Willow Walk pub in Victoria with a change of clothes for Lakatos, who went to Boodles disguised in a long dark coat, brimmed hat and long scarf.

Mr Wainwright escorted her down a glass spiral staircase into the meeting room with Ms Barton, where Lakatos examined and weighed the diamonds.

The Boodles chairman said he was “watching her like a hawk” as she wrapped the diamonds individually in pre-cut tissue paper and placed them inside opaque boxes, which were put into a zippable purse-like bag that was padlocked shut.

But Ms Barton said Anna put the locked purse into her own handbag when Mr Wainwright went upstairs to take a call from “Alexander”.

“She watched Nicholas walk up the stairs and as soon as his back was turned on the spiral staircase, she grabbed the bag and stuck it in her handbag,” she said, giving evidence.

“I said, ‘no, no, no, you can’t do that. Please take the diamonds out of your handbag now. I have to be able to see the diamonds at all times’.

“In English, she replied, ‘It’s OK, don’t worry, there’s nothing to worry about’.”

After leaving the store, Lakatos switched the diamonds to the bag of one of two unknown young women before discarding her disguise and leaving London for France on the Eurostar with Danila.

Stankovic and Jovanovic left with the two younger women in a rented car through the Channel Tunnel.

Both men were jailed for three years and eight months after pleading guilty to conspiracy to steal, while Danila was acquitted after telling a jury she had no idea she was involved in the crime.

Ioana Nedelcu, defending Lakatos, said the masterminds have never been caught.