Officers have issued a further warning to retailers and people who may handle cash as part of their work to be on the look out following a number of reports of counterfeit notes being used.
The incidents have been reported in Aberdeen and across Aberdeenshire and Moray in the last two weeks.
Typically the notes tendered are £50 notes which are labelled Bank of England or Bank of Ireland.
Investigating officer Detective Sergeant John Lumsden said: “People tendering counterfeit currency will often purchase low valued goods with high valued notes (£20 or £50) in order to receive a large amount of genuine currency in change.
“We’ve had a number of reports of this happening over the last few months and we believe the incidents reported to us so far are all linked.
“While our enquiries are continuing into these incidents I would like to encourage the public and retailers, particularly smaller independent shops, to be on their guard for this type of incident and to call police with any information they have that would help our investigation. Often the notes may have a peculiar smell as an attempt may have been made to bypass legitimacy tests.
“If anyone has concerns about money being tendered they should refuse the sale and contact police. Anyone with information is urged to contact officers on 101 or Crimestoppers on 0800 555 111 to remain anonymous."