Fake £20 notes are being circulated around the Willesden Green area of Brent, prompting police to issue a warning to businesses.
At least 14 shops in the High Road have received counterfeit notes totalling more than £500.
And officers say the cash is fooling many shopkeepers because of its genuine appearance.
The fake notes have the water-mark on them but do not have a hologram or embossed Bank of England writing.
Willesden Green Safer Neighbourhood Team has been leafletting shopkeepers to make them aware of the problem and advising them how to spot a fake note.