Thieves broke into the showroom of an Italian jeweller favoured by Hollywood stars and stole items worth up to £10 million while its owners were in Los Angeles hosting a party to celebrate the Oscars.

The heist, which took place at the Damiani showroom in Milan’s fashion district on Sunday morning, was compared yesterday with the “perfect robberies” in films such as Ocean’s Eleven and The Italian Job.

The gang had spent more than a month digging a tunnel from a disused cellar in an adjoining building. Police said that the drilling had been heard for weeks but was presumed to be part of continuing building works next door. The four thieves, dressed in balaclavas, sunglasses and fake uniforms of the Guardia di Finanza, used an internal staircase to reach the first floor, thus avoiding alarms and video surveillance cameras.

The showroom had been opened for a private viewing by wealthy clients and there were five employees inside, including a cleaning lady and a member of the catering staff, as well as the manager. The thieves said they were there to check the accounts, then overpowered the staff, tied them up with electrical cable, sealed their mouths with tape and locked them in the washroom. They then helped themselves to jewellery from the safe-deposit boxes and left the way they had come.

Police said that the entire operation had taken little more than 40 minutes. The employees managed to free themselves and raise the alarm, but by then the gang was long gone. Investigators said that they were looking into whether the robbers had had “inside assistance” that had helped them to avoid the alarms and surveillance cameras.

Damiani, founded in 1924 and known as the jeweller to the stars, is run by Guido, Giorgio and Silvia Damiani, the third generation of the family. The firm has won the Diamonds International Award 18 times, and its customers have included Brad Pitt and Gwyneth Paltrow. It supplied the diamond-studded bracelet worn by Tilda Swinton at the Oscars ceremony.

Some of the firm’s most valuable jewels survived the raid because they were on show at the Presidential Suite of the Beverly Wilshire Hotel in Los Angeles, where Damiani hosted an Oscars party at the weekend attended by celebrities such as Sharon Stone, Paris Hilton and Geena Davis.

Guido Damiani said that he was “very shaken” by news of the raid, but added that he was relieved that no one had been hurt. Police said that the thieves had not used weapons to threaten the employees and may not even have been armed.

Staff at Damiani said that the leader of the gang had a “southern Italian” accent. The stolen pieces of diamond-studded gold jewellery included necklaces, earrings, bracelets and rings. Damiani said that the total loss would not be known until an inventory had been completed, but that all the jewellery in the store was insured.

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