A sting operation in Watford town centre to check whether shops were illegally selling knives to teenagers was captured in a BBC documentary.

In episode 10 of SAS: Catching the Criminals, which aired this morning (September 20), trading standards officers from Hertfordshire County Council were filmed conducting test purchase operations in the High Street and Market Street over the summer.

A 16-year-old volunteer was tasked with going into shops and attempting to buy knives to check if they would ask for her age or identification, as it is illegal to sell them to anybody under the age of 18.

The 16-year-old volunteer with HCC officers by St Mary's Church in the town centre.The 16-year-old volunteer with HCC officers by St Mary's Church in the town centre. (Image: BBC) After one undercover attempt, the teenager said: “I picked up a chopping knife and went to the till and he was really scared.

“He asked me how old I was and I said I was 16. He told me I needed to be 21.”

The BBC filming in Market Street.The BBC filming in Market Street. (Image: BBC) They tested "several" businesses while filming in the town centre that day and found that all had complied with the law.

However, principal trading standards officer Simon Brindley said another 16-year-old volunteer was sold a set of two chef knives when the team carried out the same exercise in Hitchin “a couple of days later”.

Principal trading standards officer Simon Brindley said the incident was being investigated.Principal trading standards officer Simon Brindley said the incident was being investigated. (Image: BBC) ​“They weren't challenged for their age or ID, the premise that sold the item took no measures to prevent the sale happening,” he added.

“That matter is being investigated now and could have quite serious ramifications for that business.

An undercover 16-year-old volunteer was able to buy a knife from a Watford High Street after filming had wrapped up.An undercover 16-year-old volunteer was able to buy a knife from a shop in Hitchin after filming had wrapped up. (Image: BBC)“Obviously this is a concern to trading standards that children are able to buy knives without challenge from high street stores, because obviously we all know of the consequences that can lead to, with knives on the street and the link to violent crime.”

At St Albans Magistrates’ Court on September 11, the company behind the Premier shop, H&D brothers Ltd owned by Jignesh Patel from Arlesey, was fined £11,311 over the sale.

Hertfordshire Trading Standards said the shop had been advised multiple times due to the “volume of complaints” received, as well as being given the chance to train staff on selling age-restricted items.

Watch now on BBC1 to see our Trading Standards team in action on SAS Catching the Criminals.

Our @HCC_TS team feature as they head out on a test purchase operation in Watford town centre. #SASCatchingtheCriminals #TradingStandards #TeamWork #BBC #Documentary pic.twitter.com/8cTwLC52Da

County councillor Morris Bright said: "In this case, officers gave Mr Patel several opportunities to improve his processes, but he chose to ignore their advice.

“I’d like to commend the officers involved in bringing this case to court,” he added.

It comes as new laws are set to be introduced next Tuesday (September 24) making it illegal to own zombie knives or machetes in a crackdown on knife crime.

People with the blades have until Monday (September 23) to hand them in to police stations under a government amnesty which offers £10 or more per surrendered item.

Note: this article has been updated because a previous version incorrectly stated the successful sale took place in Watford. Hertfordshire County Council has now confirmed it was in Hitchin, which was not mentioned in the BBC documentary.