The injured guard in the Gatwick security van robbery was sprayed with an unknown substance, police said today.

The lone robber escaped with an estimated £250,000 in foreign currency in yesterday's raid.

Detective Chief Inspector Mike Alderson said forensic experts were examining clothing to see what kind of spray might have been used against the 41-year-old victim, who was also punched in the face.

He said: "I know he was sprayed but I do not know what he was sprayed with. All the clothing is being forensically examined."

He added that the investigation into the incident, which lasted a matter of minutes, was continuing today and full statements from witnesses were being taken.

The culprit, who may have been posing as a security guard, struck at about 7.30am yesterday as a blue Brinks Ltd security van made routine collections and deliveries to shops at the South Terminal.

It was parked in the South Terminal service tunnel when the robber pounced.

After punching the guard in the face, he is believed to have fled on foot.

The victim was one of a two-man security van team but was on his own in the vehicle when it was attacked, Mr Alderson said.

The second security guard, a 26-year-old man, returned to find his colleague bleeding. The victim, who has not been named, was treated at East Surrey Hospital, Redhill.

Colin Deuchars, managing director of Brinks Ltd in Britain, said around £250,000 was stolen.

He said: "It was quite a violent assault - we are still waiting to find out whether the crew member was sprayed with acid or CS gas or what it was.

"He has got a broken nose for his pains as well."