Police want to trace two youths for a string of off-licence robberies.

The pair have carried out seven raids on West Sussex stores during the past month, each time threatening the cashier with a stun gun.

All the robberies have been carried out in out-of-town-centre stores after 9pm.

The youths have escaped with up to £900.

Police have warned shop owners to be vigilant and have advised how to act if the robbers strike.

Crime prevention officer PC Peter Jones said: "Staff should really try to keep themselves safe at all costs.

"I do suggest people should not resist them. If you lock your till and remove yourself from the scene, hopefully they will leave without success."

PC Jones also called on shopkeepers to ensure security cameras were working properly.

He said: "Most off-licences have CCTV. This is wonderful but you must get good images.

"Too many stores do not replace their recording tapes often enough so the tape does not show what the camera sees because it is degraded."

Detective Inspector Jeff Riley, who is leading the investigation called Operation Blitz, has appealed for anyone recognising the youths to come forward.

The first male is white, aged 16 to 20, about 5ft 9in to 6ft, and slim to medium built. He has dark hair and wears a baseball cap, sweatshirt and dark trousers.

His accomplice is also white, aged 17 to 18, 5ft 4in to 5ft 7in. He is slim with fair hair and an angular face. He also wears a baseball cap, a sweatshirt and dark trousers.

Mr Riley said they took it in turns to brandish the stun gun, which is black and about the size of a mobile phone.

It gives off blue flashes of electricity, although no victims have been assaulted with it.

Robberies have been carried out at seven shops so far in Bognor, East Preston, Barnham, Angmering, Billingshurst and Middleton-on-Sea.

On two occasions, the robbers fled empty-handed.

Anyone who can help should contact Mr Riley on 0845 6070999 or Crimestoppers in confidence on 0800 555111.