An armed gang has escaped with up to £40 million in cash from a security depot after kidnapping a guard's young family in what could be Britain's biggest cash robbery.

The elaborate operation began at 6.30pm yesterday when a security manager driving home from a shift at the Securitas cash depot in Tonbridge, Kent, was pulled over by a car with flashing blue lights in its radiator grille.

Men in police-style fluorescent tabards and helmets ordered him to step outside before bundling him into the Volvo, which they said was an unmarked police car, and restraining him with handcuffs.

Simultaneously, two men in police uniform visited the manager's home in Herne Bay, Kent. They tricked his wife and young son into following them by telling them he had been involved in an accident.

The manager was tied up and transferred to a white van: he was told at gunpoint that his wife and son would "suffer" if he refused to co-operate.

The van was driven to the depot where at least six members of the gang, carrying handguns, stormed the office in Vale Road at around 1am. Guards were tied up as the men set about loading the piles of bank notes.

It took more than an hour for the gang to load the enormous haul into the 7.5 tonne truck before driving away. The traumatised staff raised the alarm an hour later, at around 3am. The depot was used as a storage facility for cash sent out to ATMs across the South-East.

Police in Kent refused to state exactly how much money was stolen, saying only that "millions" of pounds has been taken. The Bank of England said tonight that the amount was "more than £25 million".

But The Times has been told by government sources that the Bank of England fears the total amount stolen could be up to £40 million. That would dwarf the £26.5 million stolen in an IRA robbery on the Northern Bank in Belfast last Christmas.

Detective Superintendent Paul Gladstone: "This was a traumatic ordeal for the manager of the security depot and his family and for all of the staff who worked there. They have all coped extremely well in the circumstances, when faced with threats and a gang of armed men.

"It is vitally important that we hear from people who may have seen something suspicious or someone acting oddly either round the depot in Vale Road, or in any of the locations where this gang were operating in the time up to the robbery.

"This was a clearly a robbery that was planned in detail over time – someone must have information that will help us in the hunt for the robbers.

"A very substantial sum of money running into millions of pounds has been stolen and we are determined to bring the robbers to justice."

Anyone with any information about this incident is asked to contact Kent Police on one of two special incident room hotline numbers - 01622 652361 or 01622 652366.

Britain's biggest robberies

1963: the Great Train Robbery at Aylesford, Buckinghamshire, yielded £2.6 million, the equivalent of £40 million today

1980: silver bullion worth £3.5 million is stolen in East London

1983: a gang steals £6 million from a Security Express depot in East London

1983: gold bullion worth £26 million is taken from a warehouse at Heathrow

1987: cash and valuables thought to be worth £40 million are taken from a Knightsbridge safe deposit

1990: bonds with a theoretical value £292 million are stolen from a City messenger

1999: a gang steals £6.6 million from a bank clearing house in Manchester

2002: £6.5 million is stolen from a Heathrow security van

2003: the Pink Panther £23 million jewellery robbery in Central London

2004: more than £26 million is stolen in raid on Northern Bank in Belfast

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