A gang robbed a security guard and escaped with thousands of pounds.

The raiders are believed to have threatened the worker with a knife during the second robbery on a Securicor van in Brighton and Hove in 24 hours.

Eyewitnesses saw three men wearing scarves across their faces approach the guard as he returned to his vehicle in St George's Road, Brighton, after making a collection at the Co-op supermarket.

The robbers snatched the guard's money box from him and were chased by a member of the public to a blue Rover getaway car in Marine Parade at about 10.30am yesterday.

About 24 hours earlier three men robbed two Securicor guards outside Barclays Bank in Boundary Road, Portslade.

Detectives say it is too early to link the incidents but union leaders today warned workers transporting large sums of cash were not receiving enough protection from the authorities.

Although "cash-in-transit" robberies are rare in Sussex, they are happening increasingly frequently. There were three in 2004 and that number doubled last year. But there have already been three attacks on Securicor workers this month.

The GMB union says as the banks have increased their internal security to protect their customers and staff, the problem has been pushed out on to the pavement and cash in transit vans and couriers are increasingly under attack as they make deliveries.

Acting general secretary Paul Kenny said: "The attacks are now so commonplace that they rarely warrant media attention unless there is a spectacular robbery.

"They are a hidden crime - unless a police officer or a member of the public is involved, these attacks only warrant a few lines in the Press.

"But our members, men and women whose lives are equally precious to their families, are being subject to horrifying attacks when carrying out their day to day work."

The GMB is calling for police to target the organised criminal gangs it believes are behind most of the robberies, stricter sentences for armed robbers and the removal of parking restrictions banning security vans from town centres.

Police were making door-to-door inquiries yesterday in the area around the latest attack.

Detective Inspector Ian Pollard, who is leading the investigation, said: "We are extremely grateful to the member of the public who chased after the offender but we need assistance from other people who may have witnessed what happened or who may know the whereabouts of the car.

"We ask the public to not approach the vehicle but contact us immediately."

Police figures show robbery is the least common form of crime in Brighton and Hove, accounting for just one per cent of all offences.

A spokesman for G4S, which runs Securicor, said: "G4S Cash Services (UK) is committed to maintaining the safety of its staff and the public.

"As such, we are in regular discussions with trade unions, the police, the Government and local authorities to develop initiatives that will curtail the growing number of attacks taking place on the cash-in-transit industry."

The men were wearing dark clothes with hooded tops.

Anyone with information should telephone 0845 6070999 and ask for Detective Sergeant Glenn Taylor.