Forged Scottish £20 notes are in circulation in the south Lancaster area, police said.
They are green in colour and local shops have been advised to check all notes.
If anyone believes they may have been handed one, then please go to your nearest bank to hand this in and to give details of how they have obtained it and where.
It is an offence to try and use counterfeit currency or to pass it on knowing it is counterfeit.
Local PCSOs are checking CCTV in shops and stalls of anyone trying to pass over forged notes.
Please contact police on 101 with any further information.