UP to £40m was stolen from a security depot in what is thought to be Britain's biggest robbery, it emerged last night.

The Bank of England said at least £25m had been stolen, but sources said the true figure could be as high as £40m, making it the biggest theft in British history.

The Governor of the Bank of England called for a review of security arrangements for the storage of banknotes following the raid on the Securitas cash depot in Tonbridge, Kent.

The raid happened at about 2.15am, when 15 members of staff were threatened and tied up.

The gang posed as police and abducted the depot manager, his wife and son, and threatened him at gunpoint, Kent Police said.

Detective Superintendent Paul Gladstone, of Kent's serious and organised crime unit, said the raid was "clearly a robbery that was planned in detail over time".

He described the figure stolen as "a very substantial sum of money running into millions of pounds".

The raid at the Northern Bank's Belfast headquarters in December 2004 netted £26.5m, making it the biggest cash theft in British and Irish history at that time.

The manager of Securitas' main cash depot south of the Thames was pulled over while driving near Stockbury, Maidstone, at 6.30pm on Tuesday, by what he believed was an unmarked police car, Kent Police said.

A man wearing a high-visibility jacket and police-style hat got out of the vehicle, which had blue lights in the radiator grille, and spoke to him.

Thinking they were police officers, the manager then got into their car -where they handcuffed him.

Meanwhile, two more fake policemen visited the manager's wife and young son, told them the manager had been involved in an accident, and took them from their Herne Bay home, officers said.

The manager was put inside a white van and taken to an unknown location, where he was threatened at gunpoint and told to "cooperate or his family would be at risk".

At about 1am yesterday, he was taken to the Vale Road security depot where least six men, some masked and armed with handguns, threatened and tied up the staff.

The gang loaded the cash into a white lorry before driving off at about 2.15am.

Police were alerted about an hour later when the shocked but uninjured staff activated an alarm.

The manager, his wife and son were also unhurt.

A Bank of England spokeswoman said: "We have already been reimbursed by Securitas for the initial estimate of £25m and any further sum will be reimbursed to the bank as soon as the amount is known.

"There is no cost at all to the bank or the taxpayer.

"The governor today asked for a review of the security arrangements for the storage of banknotes."

A Securitas spokesman said an audit to work out how much had been stolen would take place through the night.

The firm's operations director, Tony Benson, said: "On behalf of Securitas, I would like to express our revulsion at the circumstances of this particular crime.

"Members of our staff have had inflicted on them the most terrible and traumatic experience - something no one going about their everyday job should be subjected to.

"Our thoughts are with the staff affected and, of course, their families."