FORGED £20 notes are flooding into Huddersfield.
Eagle-eyed shop owners have spotted some of the fakes - but the tricksters have been spending them at the town's pubs, clubs, shops and fast-food restaurants.
Now people have been urged to check their notes carefully and they need to be sure they are genuine.
One man was arrested yesterday on suspicion of using the fake money.
One good check is to hold them up to the light and see if small dark marks run across the metal strip. They are missing from many of the fakes which have a different texture.
Police believe fake notes are being sold.
Sgt Tim O'Sullivan, of Huddersfield police, said: "There are a lot of forged £20 notes around at the moment and we would urge people to be exceptionally vigilant.
"The people using the money often buy something of low value to get as much genuine cash back as they can on each transaction.
"We would urge bar staff and shop assistants to check the notes carefully. The notes just do not feel right."
A teenage girl tried to spend a note in Marsh - but was rumbled immediately by Alison Rayner, owner of card and gift shop Don't Forget.
She said: "The girl came in on Tuesday afternoon and after browsing around for a while tried to buy a £1.50 notebook with a £20 note.
"As soon as I handled the note I knew it was a fake. The texture was totally different. It had not been printed on the right paper and the colours looked pale. The lines were also missing across the band when I held it up to the light."
Mrs Rayner said she got a genuine £20 note from her till and was about to show the girl the difference between the two notes when the teenager grabbed the fake note back and left the shop.
Alison alerted other shops throughout Marsh on a retailers' Shopwatch radio system. The girl was then reported to have been thwarted from using her note at the nearby Co-op supermarket.
She was last seen walking down Broomfield Road.
Police are still trying to identify her to find where she was given the cash and a description has been issued.
She was about 17, 5ft 3ins tall and very slim with blonde hair. She wore pale blue jeans and a powder blue jacket.
Sgt O'Sullivan said: "We know different people are using the notes throughout the town."
Contact police on 01484 436659.