Gangs of raiders are getting away with at least one gunpoint robbery a month in Sussex.

Since January last year, just five of the 28 robberies on businesses involving firearms have been solved – leaving 23 in the past 20 months unsolved.

Attacks on security vans delivering and collecting cash in the South East have also soared in the same period, despite falling nationwide.

The latest figures from the British Security Industry Association reveal there were 30 cash-in-transit robberies between January and June this year, compared with 17 in the same period last year, a rise of 76%.

At the same time, there was a 20% fall in the number of cash-in-transit raids across the rest of the country.

Detectives in Sussex privately admit that in some cases their best chance of catching raiders is if they strike again.

Victims of gunpoint robberies said the low number of successful police investigations made them feel more vulnerable to attack.

Rushanehan Thevaraju, the manager of the Bristol Estate Convenience Store in Donald Hall Road, Brighton, which was robbed on July 28, said: “I used to bring my son in who is two and a half but I don’t feel it is safe for him now.

“It is really worrying.”

In October last year, 76-year-old grandmother Balwant Saimbi needed hospital treatment after robbers carrying a gun hurled a garden chair at her at South Street post office in Portslade.

Postmistress Charanjit Dhajan, who was also in the shop at the time, said they are both still feeling the effects of the robbery.

She told The Argus: “It has affected our confidence. I wish I didn’t have to work here. The only reason I’m doing it is because I don’t know what else to do.”

High-profile robberies which are still unsolved include raids on cash deliveries to the Nationwide building society in Church Road, Hove, last month and in March last year.

Police believe many of the armed robberies in the county are being committed by gangs from London and Liverpool.

A string of robberies on cash delivery vans in Brighton’s Kemp Town area, Hove and Portslade were carried out between 2006 and 2008 by members of a Merseyside gang.

Alan Mills, Anthony Conning, Anthony Gill and Jamaal Alfa are due to be sentenced for the attacks on September 11.

Three men from Liverpool were arrested after a robbery from a security guard at Tesco Express in Woodingdean in May.

Detectives say a crackdown on similar crimes in Merseyside has driven gangs to target other areas of the country.

James Reilly, 45, from Wandsworth in London, was arrested in the street by an off-duty police officer after robbing Royal Bank of Scotland in Castle Square, Brighton, in February.

He had previously struck in Tooting and Streatham. In each robbery he passed a note over the counter claiming he had a gun.

Three men from London are due to appear in court today on robbery charges over a raid in Crawley which ended in a dramatic car chase in May.