The raid took place in a zone covered by CCTV cameras, raising suspicions that the robber was an opportunist.
Detectives are also considering the possibility that he had been given "inside help" by an employee at Gatwick.
The money was taken from a van owned by Brinks Ltd, formerly Brink's Mat, the company that lost £26m of gold bullion during a raid on a security depot at Heathrow airport in 1983. That incident took place "airside" - inside the perimeter fence of the airport, but Gatwick said yesterday's robbery had taken place in a public area and security had not been breached.
According to Sussex police, the raid took place at 7.30am in an access tunnel underneath Gatwick's south terminal that is used to bring supplies to some of the airport's 140 shops.
Detective Chief Inspector Mike Alderson said the van had made a routine stop and one of the guards had got out to collect cash from the stores. Although police said details were unclear, it seems the robber, who was dressed in clothes that resembled the Brinks Ltd uniform, opened the van's side-door, punched the remaining guard in the face, and fled carrying a "significant" amount of cash.
The guard received a broken nose and was taken to East Sussex hospital.
"No weapon was used," said Mr Alderson. "This occurred in an area covered by CCTV. We are studying that footage, talking to witnesses, and talking to the two security guards. We are investigating the possibility the offender was posing as a security guard."
Mr Alderson said that the security of the airport had not been put at risk at any stage.
"The van was parked in a public area. Gatwick is a controlled anti-terrorist environment, and this type of incident is highly unusual."
