Shopkeepers are being warned to be on the lookout for "convincing" forged £20 notes.
Police said during the last ten days officers had received several reports of people trying to use the fake notes in Middleton, all of which have the same serial numbers.
Officers are reminding shopkeepers to check all notes which are handed in and report any suspicious money to the police.
The notes which have been recovered all bear the serial number PF20695103.
Sergeant Simeon Holt said: "These notes are very good quality and would pass for real ones just by looking at them.
"I would remind shop owners to check every note they take under ultraviolet light. If you cannot do this, make a note of these serial numbers and check them against the money you are accepting.
"Anyone who's selling anything privately should also be aware that these notes are in circulation.
"If anyone does come across these notes I would ask them to hand them in to their nearest police station."
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