Shoppers and retailers in Cheam have been urged to be on the lookout for fake £20 notes after two shops - including a charity shop - were caught out.
Police are hunting two suspects after they handed fake notes to workers in Save the Children in The Broadway and Tasha's Tea Room in the High Street last week.
But officers have warned people to look out for the tell tale signs that notes are fake in order to prevent them falling foul in the same way.
In both instances, two £20 notes were handed over to pay for low-value goods. Despite checks, they were accepted and the errors were only revealed when the money was taken to the bank.
Ray Kasey, chairman of the Sutton and Cheam branch of Save the Children said: "Obviously we're very disappointed that people chose to use a charity shop like this - it's effectively stealing.
"Just to pick an example, that £40 could have bought live-saving antibiotics to treat four newborn babies too weak to fight infections themselves."
The two suspects are both white males in their late teens. One has a distinctive round face.
Police have advised people to always check notes to make sure they are genuine, Tips to help people tell if their note is real include:
- run your finger across the front of the note to feel raised print in areas such as the words ‘Bank of England’
- hold it up to the light to see an image of the Queen's portrait and that the metallic thread appears as a continuous line
- check that colours are sharp, clear and free from blurred edges.
- Driver cautioned after Rosehill crash puts two pregnant women in hospital
- PICTURES Terror alert - Fright Night at Thorpe Park for Halloween
- EXPLAINED: The new 105 phone number and what else you should do in a power cut
- Police searching for missing woman in Michael Jackson t-shirt
- 'Long term future' of nurseries could be at risk from proposed flat-rate funding, London Councils warns