The money had arrived on a South African Airways flight at 7.30am today. It was transferred to the security van, which was then stopped by two men who held a knife to the driver and forced him to drive out of the airport.
The van stopped in neighbouring Cranford, where the men transferred the cash to their own vehicle before disappearing, police said. The security van was abandoned and the driver left unhurt.
The two hijackers were both Asian and thought to be in their 20s. The Flying Squad was awaiting confirmation of the exact amount of money stolen.
In another development, police said that they had arrested a man at 6.30am today in London in connection with a multi-million-pound theft from Heathrow last month.
On February 11, thieves successfully targeted another consignment of cash at a time when security at the airport is supposed to be watertight after the terrorist attacks of September 11.
The thieves got away with £4.6m worth of foreign currency last month. The transport secretary, Stephen Byers, who is responsible for aviation security, and the home secretary, David Blunkett, demanded an "urgent report" into the implications of the robbery during a time of heightened security.
In last month's raid, two robbers armed only with plastic wrist ties attacked the driver of a British Airways security van in a secure airside cargo loading area near Terminal Four.
Police said that it may have been an inside job.
The men escaped in another British Airways van with eight boxes of cash, which had just been taken off a jet from Bahrain.
