The Bank of England confirmed that “more than £25 million” was taken. The raid was so big that senior ministers and officials at the Bank were alerted within hours.

The gang struck in the middle of the night at one of 11 cash management centres run by Securitas, with some of Britain’s biggest banks, to service the country’s ATM system.

The gang breached the sophisticated defences of the depot in Tonbridge, Kent, by forcing the manager to take them inside after kidnapping and threatening his wife and small son. In little more than an hour the gang had tied up the 14 staff, loaded a 7.5-tonne lorry — and disappeared.

Last night bank officials and senior staff from the Securitas company were still waiting for police to allow them to audit the losses. But The Times has been told by government sources that the police and the Bank of England fear that the amount stolen could be as much as £40 million.

Last night Securitas said that at least £25 million of the cash belonged to the Bank of England, and that the firm had already reimbursed them for the loss. The Bank of England said that the cash had been held by Securitas on behalf of the bank for distribution to meet consumer demand, and was a mix of new and old notes. The Bank added in a statement that there would be no cost to it or the taxpayer.

As forensic science officers worked at the depot in Vale Road, police sources admitted that “multiple millions of pounds” had been stolen. Such a haul would make the raid the biggest cash robbery in British history, dwarfing the attack by the IRA on the Northern Bank in Belfast last year and the £26 million in gold bullion stolen in the Brink’s-Mat robbery more than 20 years ago.

If the final sum is £40 million, it will be the second biggest heist ever, shading the £38.8m stolen from a branch of Brazil’s Central Bank, in Fortaleza, last August.

However none compare to the £516 million taken in March 2003 from the Iraq Central Bank, shortly before the fall of Saddam Hussein.

Detectives are scanning intelligence files and putting out feelers in the underworld for information on the gang behind the raid. The operation, known among criminals as a “tiger kidnapping”, would have taken months of surveillance of the depot and its staff and required split-second timing.

Police will be hoping that a record reward will tempt informants. Insurance companies and loss adjusters usually offer 10 per cent of the haul, making a possible reward of £4 million or more.

The depot is part of a network run by Securitas, Europe’s biggest cash transportation company. Four years ago it set up a “bulk cash management” company, with the help of HSBC and Barclays. Instead of individually transporting and storing money, the company gave the banks the opportunity to cut costs by using only a few anonymous sites to store cash, protected by the most sophisticated security equipment.

The Bank of England’s cash storage depot, for example, used to be just off the M11 near Loughton, where nearby roads were designed to stop thieves making a quick getaway.

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