No one was reported injured.
The cash, the equivalent of about $43 million, was stolen early on Wednesday from a depot at Tonbridge, Kent, a spokesman for the Bank of England said on customary condition of anonymity.
Original estimates indicated that 1 million pounds was stolen, but by Wednesday evening, the estimate had risen and authorities said it was one of the country's biggest robberies.
The 20 December robbery of 26.5 million pounds (at the time, 38 million euro, or $50 million) from a bank in Northern Ireland was called the biggest cash theft in history.
Dressed as police
Wednesday's robbery took place after the cash depot closed when the thieves dressed as police officers stopped the manager of Securitas Cash Management as he drove home.
Detectives said the manager got into their car, which he thought to be a police vehicle, and was handcuffed by the robbers.
At the same time, another team of thieves went to the manager's house, telling the family that the manager had been in an accident. The men convinced his wife and young son to leave the home and go with them.
The first team, with the manager in the car, met up with a white van and another group of thieves. The manager was placed in the van, which then headed towards the Tonbridge depot, police said. The manager was allegedly told to cooperate or his family would be hurt.
Detective Superintendent Paul Gladstone said he thought the robbery was planned down to the last detail.