Police admit they didn't respond to huge London jewel heist

Police in London have admitted they failed to investigate a burglar alarm which apparently went off during a heist worth up to $US200 million ($A260 million) in the city's diamond quarter.

The raid in Hatton Garden, the centre of London's jewellery industry and home to hundreds of shops, is one of the biggest in British criminal history and happened over last week's Easter holiday weekend.

Thieves climbed down a lift shaft and used an industrial drill to bore their way into a vault at Hatton Garden Safety Deposit, where many jewellers had stored their stock.

Once inside, they broke into up to 70 safety deposit boxes. Media reports suggest the criminal gang may have hidden inside the building on Thursday, only emerging after staff went home, although the crime was only discovered on Tuesday after the holiday.

Scotland Yard issued a statement on Friday revealing that it received a call in the early hours of last Friday from the company operating the building's alarm telling them that it had gone off.

"A grade was applied to the call that meant that no police response was deemed to be required," police said, adding they were now investigating that decision.

"It is too early to say if the handling of the call would have had an impact on the outcome of the incident," the statement added.

Scotland Yard's Flying Squad, which investigates heists, have not confirmed the value of the burglary and many jewellers are still waiting to learn if they had items taken.

But British media have reported that as much as STG200 million in diamonds, other jewels and cash were taken.