An eagled-eye off duty police officer was behind arrests in connection suspected counterfeit £20 notes in King's Lynn.
Police were called to the town centre after being alerted about suspicious activity by the off duty colleague on Friday afternoon.
Two men were arrested and a quantity of suspect notes was seized.
Police have previously issued warnings to businesses to be vigilant against people passing off fake notes.
Retailers are asked to use crime prevention tactics including UV lights or testing pens to establish whether notes are genuine.
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