Traders and residents have been warned of a spike in counterfeit £20 bank notes in circulation in the county.
Lancaster is understood to have been particularly affected by the fake notes.
Trading Standards said some of the notes have already been recovered by the banks and by Lancashire police.
However, the authorities are appealing to residents to “take extra care” when handling cash so that any remaining notes can be taken out of circulation. Those handling cash on a regular basis are being asked to make members of staff and colleagues aware.
Many of the notes that have been seized by officers are described as having a yellow tinge to the Queen’s head, but a genuine bank note will have a black tinge around the Queen’s head.
If anybody believes they have found or are in possession of any counterfeit £20 notes, they should contact their bank and police by calling 101.
The Bank of England estimates 0.0053 per cent of notes in circulation are counterfeit.