The robbers abandoned the van in an underground car park

He then let two accomplices into the vehicle.

They drove to an underground car park, where four steel cases containing the cash were transferred to another vehicle, say police.

The handcuffed guard was discovered only when passers-by heard him calling for help. He was not injured, a police spokesman told BBC News Online.

The co-driver is helping the police with their investigations, but has little information about the driver.

He did not know him personally as the security firm constantly changes its staff, police say.

It is thought that the two accomplices may also have been foreign.

Police have yet to identify the getaway vehicle.

Frankfurt police said they would engage the help of other police forces in the investigation, through Interpol.

It is thought to be the biggest euro raid carried out since distribution of the new currency started last year.

The massive distribution operation involved supplying around 600bn euros to banks and shops across the 12 eurozone countries.

The first major hitch came in November, when armed robbers escaped with 260,000 euros (�160,000) in new banknotes, after holding up a bank in a Rome suburb.