Mervyn King, governor of the Bank of England, began an urgent review on Wednesday night of security arrangements for storing banknotes after the audacious raid on a cash depot that could turn out to be Britain’s biggest ever bank robbery.

The raid, in the early hours of Wednesday morning, was at a Securitas depot in Tonbridge, Kent, one of about three dozen run by private companies and used by the Bank to distribute notes around the country.

Only a few weeks after the post-Christmas sales, said by officials last night to be a peak period for transporting cash, the centre was filled with what was probably a mix of new and used notes, worth at least £25m and as much as £40m.

The robbery appears to have been a sophisticated and well-planned operation, with the raiders, posing as police officers, kidnapping the depot manager’s wife and young son from their Herne Bay home.

According to police in north Kent, the depot’s manager was stopped on his way home at about 6.30pm on Tuesday near Stockbury by what was thought to be a Volvo with blue flashing lights in the radiator grill.

The two occupants, one of whom was wearing a high- visibility jacket, appeared to be police officers. One of them spoke to him and, thinking they were genuine officers, he got into their car. He was handcuffed and driven along the M20 to West Malling.

The car later met up with a white van and the manager was tied up and put inside the van.

He was taken to an unknown location, threatened at gunpoint and told to co-operate or his family would be at risk.

At about 1am he was taken to the Vale Road security depot, where at least six men, some of them armed, tied up 15 staff and loaded the cash into a white truck. The gang drove off just over an hour later.

Bank officials made clear that any losses would not be borne by the taxpayer. Securitas, which is expected to rely on insurance cover, had already reimbursed it with £25m and more could follow as the amount stolen was verified.

The company is one of five used by the Bank to distribute notes. The others are the Post Office, Royal Bank of Scotland, HBOS and Securicor. The Bank has two distribution centres, one in Leeds and the other in Debden, Essex, at its printing works.

The logic behind outsourcing distribution to private operators and numerous storage sites is that having these centres closer to the retail banks that use the notes reduces the risk of robberies of vehicles, which over time are more frequent.

The only previous theft at a storage centre was a much smaller one of £100,000 in cash at Debden 15 years ago. Bank officials said there was no agenda to its security review, but acknowledged that concentrating cash in depots, while reducing the risk of vehicle robbery, left it vulnerable to a giant haul.

The amount of notes in distribution varies throughout the year. What is clear, though, is that the robbers struck just as a lot of cash was in circulation, after the peak Christmas spending period. Storage depots would have been full.

The gang now faces the challenge of hiding, and ultimately spending, its huge haul without being caught. Former police officers said the fact that much of the money was in used notes would make the gang’s task easier because the cash would be more difficult to trace.

With new banknotes, the authorities can keep track of the serial numbers to find out where and when the cash is used. Gold bars can also be traced unless they are melted down – a difficult and time-consuming process.

Nevertheless, simply moving and storing such a huge sum of cash could be difficult, as banknotes take up a large amount of space.

Regulations designed to stop money laundering mean it is almost impossible to deposit significant sums of cash in a bank account without explaining where it came from and alerting the authorities. Some gangs have used other institutions, such as foreign exchange bureaux, to convert the cash into other currencies before transporting it out of the country.

Much will also depend on the sophistication of the gang and whether it is prepared for such a long wait.

History suggests that participants in big bank robberies find it difficult to resist the temptation to show off their new-found wealth, potentially alerting other underworld figures and police informants.

One of the largest bank robberies in recent years took place in December 2004, when a gang stole more than £26.5m from the headquarters of Northern Bank close to Belfast City Hall in Northern Ireland. The gang, suspected to have been IRA members, is alleged to have broken into the homes of two bank staff and kidnapped their terrified families. The employees were coerced into handing over a large sum of money from the bank’s distribution centre.

Additional reporting by Jane Croft

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