POLICE have launched an investigation after teenagers went on a sham shopping spree, ripping off charity shops with fake £20 notes.

The youths targeted at least three charity shops in Wantage on Friday afternoon, palming off three notes and taking nearly £60 in change.

Now the issue is a growing concern.

Michael Butler, who runs the Mind shop in Market Place, said the two teenagers came into the store within an hour of each other, both brandishing suspicious Scottish notes.

His volunteer Diana Gawlas checked the notes with a special pen to expose forgeries, but they both passed the test.

After it happened the second time, she raise the alarm with Mr Butler, who took them to the bank where a teller confirmed both as forgeries.

Mr Butler, who lives in Grove, said: "It's despicable.

"One guy came in with a £20 note, picked up something that cost a pound and paid for it with this note.

"My volunteer said it didn't feel quite right and she wasn't sure about it but it passed the pen test.

"Then an hour later another lad came in and did the same thing, so they both took £19 in change."

After his visit to the bank, Mr Butler went to warn the other charity shops only to discover they had already been hit.

The fakers had targeted the nearby Helen and Douglas House charity shop, palming off another fake Scottish £20 on them, and also the Shaw Trust shop, but volunteers there had been too suspicious to take the fake.

A volunteer at the Helen and Douglas House shop said they had put their fake £20 up on the wall to raise awareness.

She added: "We've had so many comments – people are sickened by it, and keep saying 'why do they target charity shops?'"

Mr Butler said after he had reported the notes to police on Friday night he got a call from an officer in Didcot saying the town had had similar problems.

He said: "The banks have said they've had it a lot as well – I think this is a bigger problem."

Thames Valley Police confirmed it was investigating.

Spokeswoman Hannah Jones said: "We received a report that two Scottish counterfeit notes had been exchanged by two people on Friday in Market Place, Wantage.

"We are investigating the incident of forgery and if anyone has any information about it they should contact Thames Valley Police's enquiry centre on 101."