The two masked men, armed with sawn-off shotguns, are believed to have crawled under the Security Express van as it stopped at a roundabout off the M6 near Carlisle on Wednesday night. After the van drove into the Armaguard depot in the town's Kingstown area, the men leapt out and surprised staff. They fled with the money in purple cloth bags with the National Westminster Bank logo on them.
Police were alerted by an alarm system but the raid took only a matter of minutes and the robbers were driven away by a third man in a stolen van before officers arrived at the scene.
Detectives think the van could have been forced to stop at the roundabout deliberately by a red saloon car driven by a grey- haired man in his fifties.
As the robbery was taking place, a white Transit drove into the depot. Two gunmen loaded the money into it and made their escape. The van was later found abandoned near by.
Detective Chief Inspector Ian Douglas said: 'It was a well planned and executed robbery - but I am not praising these people, they had guns and might well have used them.' Police said Armaguard was offering a 'substantial' reward for information.