Armed thieves escaped with thousands of pounds in cash after a raid on a Tesco store tonight.

The gun-wielding gang pounced on a security guard as he attended to a cash dispenser just before 7.30pm at the Tesco Express at the Hathaway Retail Park in Chippenham.

Two men bundled the guard, who is in his early 40s, to the ground as he left a secure room in the store.

They raced out to a waiting green Volkswagen Passat, driven by an accomplice, and sped down down Malmesbury Road. The car was driven to an access road off nearby Greenway Lane and set alight before the gang leapt into another car and escaped.

The narrow access road leads from Greenway Lane out to the Lyneham Road, just minutes from the M4.

Sergeant Martin Alvis said the security guard was unhurt by the attack.

"He was very shaken but he is okay. He was checked over by a paramedic and he was able to give us a very good statement afterwards.

"There were three members of staff on duty at the shop and they were also obviously shaken by the raid. But they have been spoken to by their area manager and are all okay.

"The raid was all over in a matter of seconds and it has been very upsetting for them. Fortunately no one has been hurt."

Sgt Alvis said the staff believed one of the raiders was carrying a pistol.

"We are investigating reports that one of the men was carrying a small firearm," he said. "Obviously we would like to hear from more witnesses on this."

He said it is not yet known how much money the raiders had got away with. "But we are talking about a substantial amount."

The guard was working with a partner who was waiting for him in a van parked outside the shop.

It is not yet known how the gang knew the guard was at the store, although it may have followed the security van before the raid.

Earlier in the day witnesses reported seeing four men wearing black hooded tops in a BMW outside Redland Stores. They waited for a short time before speeding off.

Shocked neighbours in Greenway Lane were alerted by an explosion as the raiders set fire to the dumped Passat just after 7.30pm. The car was abandoned in an access road just opposite the Co-Op convenience store. The raiders had filled the abandoned car with fireworks before setting it ablaze.

Susan Ibrahim, 43, of Greenway Lane, heard the bang as the car went up in flames while she was watching television. She was called out by Lisa Wilson, 24, who lives in the flat above her.

"I heard the bang and then Lisa called me outside. It was in the access road that goes down the side of our flats. There are some garages down there.

"At frst we thought someone was in the car and it had hit something," said Miss Ibrahim, who has only been living there since last May "There were flames all round the car, some of them must have been ten feet high. And there were fireworks going off as well. It was quite frightening."

Miss Wilson said the raiders must have had local knowledge. "Only someone who lives around here would know that this access road goes out to the Lyneham road," she said. "It is such a little road you can't see where it goes when you drive past."

Scenes of crime officers inspected the car before it was taken away for further forensic tests.

Crime Scene Investigation officers were at the Tesco store, as well as uniformed staff keeping cars away. The shop is due to reopen on Tuesday morning.

Police are appealing for witnesses to the raid to come forward.