A police spokesman said that a number of fake £20 pound notes are being circulated in and around Pendle and that residents should check for these fakes by looking at the serial number and the watermark.
The fake notes all have the serial number SY75 843008 which is printed on the back of the note.
Police are also urging residents to check the watermark as the fake notes have a very faint image of HM The Queen's head.
"Do not accept these notes," a police spokesman urged. "If anyone tries to use these to purchase anything from you remember you have the authority to seize them without exchanging or supplying goods or services and present these to the police for destruction.
"Trying to use these notes in person or through a machine is a criminal offence and anyone caught doing so will be arrested and prosecuted by the police."