Counterfeit currency has been spotted at a raft of coffee shops, supermarkets, a fast food outlet and at Queen Alexandra Hospital.
Now police investigating the ‘despicable’ crime have released CCTV of two suspects from two incidents they want to speak with.
Cosham and Paulsgrove neighbourhood officer PC David Ronchetti is leading the investigating.
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He said offenders were using the fake notes to buy low-cost products so they received a large amount of genuine change in return.
‘It’s a very despicable crime. Fraudulently using money to defraud local businesses, some of which are small independent businesses has quite a significant impact,’ PC Ronchetti said.
‘It’s effectively stealing the product that they’re buying with the money.
‘We’re reminding people to be vigilant about this kind of crime.
‘You might get that as your change, so it might impact other customers.
‘There’s been a big spike in the last couple of months through July and August.’
PC Ronchetti is now probing several incidents to see if they are linked.
He added: ‘We take a very stern view of this type of crime.
‘Offenders can expect a knock on their door very shortly.’
Incidents where fake £50s have been used or attempted to be used were:
n McDonalds in Cosham on August 5.
n The Sandwich Shop in Cosham high street, on August 4.
n Everetts Pharmacy on July 26.
n Costa, Amigos and the Costa kiosk at Queen Alexandra Hospital on July 23.
n Timpson on July 10.
n Krispy Kremes on July 10.
n Tesco, North Harbour, on July 10.
n Costa in Cosham High Street on July 7.
Police say coffee shops are targeted as they have a high number of customers, with staff quickly handling cash.
In some incidents an individual has tried to use the cash but when challenged, others, thought to be accomplices, got involved, telling staff they had seen the suspect withdraw the cash from a bank machine.
The spike in fake £50 notes follows police spotting similar trends in previous months of fake Scottish notes and counterfeit £20 notes.
Sgt Rob Sutton warned anyone caught with fake notes would be facing a prison sentence.
The maximum sentence for tendering or passing counterfeit cash is 10 years.
‘If there’s the evidence it’s imprisonment,’ Sgt Sutton said. This isn’t a slap on the wrist or a fine for committing this.
‘This is imprisonment.’
Officers have been handed some of the notes from the incidents, but are urging workers and firms to report the incidents to police.
They urged workers to never hand the money back to the suspect, and to pass over the notes as evidence to police.
While PC Ronchetti is investigating the incidents in the Cosham area, there are reports of fake £50 notes being used in Whiteley.
Sgt Rob Sutton said: ‘People are buying low-value items to get the most amount of change back — and from a £50 note they’re getting a lot of real currency back.
‘The big companies, multinationals, it doesn’t hit them in the pocket as much as for our local retailers.
‘This is actually causing them significant loss.’
He added: ‘We’re working with the Bank of England and the Portsmouth Business Crime Reduction Partnership (PBCRP) and local retailers to try to deter these crimes from Portsmouth.’
A spokesman for PBCRP said it had warned its businesses of the problem but none had accepted fake notes.