Bookmakers have been put on red alert after a series of armed robberies in which thousands of pounds were stolen.

Three bookies, each likely to have been laden with cash after the busy Cheltenham Festival, were targeted by knife-wielding raiders on Sunday and Monday.

A police spokesman said: "It is very rare for three robberies like this to take place within 24 hours. There are a number of investigations going on but we are not ruling out the possibility they are linked."

The first robbery took place at William Hill in Surrey Street, Brighton, at about 11am on Sunday.

A man produced a knife and forced a member of staff to hand over about £400 in a canvas bag.

He fled on foot towards Brighton station, sparking a search involving the police helicopter and dozens of officers.

One onlooker said: "There were about eight police cars here all of sudden and it seemed to go a bit mad for 30 minutes or so.

"The helicopter was buzzing about overhead and they seemed to be searching everywhere for him."

William Hill said a member of staff, who had just opened the shop on Sunday morning, was very shaken and was being offered counselling.

The robber is described as black, aged between 20 and 30 and about 6ft.

He wore a bright red jacket with "west coast" written on it.

A member of staff from the adult entertainment Private Shop on Surrey Street believed the robber could have been in his shop only minutes before the raid took place.

He described the man as very polite and said he browsed the shop before walking out.

He said: "If it was him, and I'm not certain it was, he could have been sizing us up. We've never been robbed since we opened so it could have been a lucky escape."

In a second robbery, a man also made off with a substantial amount of cash from a Coral branch in Rustington.

The raid took place yesterday at about 10am.

A man brandished a small kitchen knife and threatened a lone female member of staff in the Sterling Parade shop.

He grabbed money from the till and ran off.

One nearby shop owner said: "I can't remember the last time someone was robbed around here.

"If it was payout day for Cheltenham and there was only one member of staff. She's a very nice girl and we feel sorry for her."

The female victim, who is in her 30s, was shaken by the raid.

A "closed" sign was hanging in the shop's window yesterday afternoon.

Sussex Police described the robber as black, about 30 years old and wearing a dark blue cap and dark blue sweater.

The third raid took place at William Hill in Freshwater Parade, Horsham, at about 11am yesterday.

No money was stolen but staff were shaken by the incident. The shop's manager was being interviewed by police yesterday afternoon.

A Sussex Police spokesman said: "In each case we are liaising with the companies involved and advising how security can be beefed up to prevent this sort of thing happening again.

"It is very much down to each shop to take precautions but we are advising where we can."

The robberies are the latest in a string of raids on bookmakers in Sussex.

Police are also investigating thefts in Ladbrokes shops in Carden Avenue, Brighton, Arundel Road, Kemptown, and Montefiore Road, Hove, since December 2005, which officers believe are linked.