Postmasters have said the allegedly counterfeit stamps were bought from Royal Mail directly, prompting fears they are mistakenly being identified as fake, the Daily Telegraph reported - adding people had complained they had to pay £5 penalties as a result.

Post Office minister Kevin Hollinrake told the newspaper: "We spoke to Royal Mail and they are investigating and they are working alongside the Post Office and other retailers to try and ascertain the source of the problem."

The issue has emerged since the postal service switched entirely to the new barcoded system in July 2023.

Royal Mail recently said its processes are "secure" and it uses "specialist equipment" to assess whether a stamp is genuine.

Most stamps are verified using the service's scanning devices at sorting offices - and it said through the added security features of barcoded stamps it has been able to significantly reduce stamp fraud.

Royal Mail said: "We want our customers to buy stamps with confidence. We strongly recommend that customers only purchase stamps from Post Offices and other reputable High Street retailers, and not to buy stamps online - unless from the official Royal Mail shop."

It added: "We are working hard to remove counterfeit stamps from circulation. We regularly monitor to detect suspicious activity, such as sales of heavily discounted stamps and work closely with retailers and law enforcement agencies to identify those who produce counterfeit stamps."

The Post Office said it takes any claims counterfeit stamps have been purchased at one of its branches "extremely seriously".

"The implication of such an allegation is that one of our postmasters, or a member of their staff, has obtained fake stamps and have chosen to sell them to customers rather than selling legitimate stamps that have come from Royal Mail's secure printers," a Post Office spokesman said.

He added: "We welcome the recent meeting between postal affairs minister Kevin Hollinrake and Royal Mail to discuss the issue of 'counterfeit stamps' and that Royal Mail have reaffirmed their close work with retailers and law enforcement agencies, and that they actively seek the prosecution of those who produce counterfeit stamps."

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Separately, Royal Mail has outlined plans to cut second-class letter deliveries to every other week day.

It said if the plans were approved by Ofcom it could lead to up to 1,000 job cuts.

In its submission to the regulator's consultation on the future of the universal postal service, Royal Mail said second-class deliveries would be scrapped on Saturdays, though it would keep a six-day-a-week service for first-class mail.

It said its proposals, which also include extending the delivery speed for bulk business mail to arrive within three days instead of two, would save it up to £300m a year.

Daily delivery routes would be cut by between 7,000 to 9,000 within two years as a result, which would likely lead to job cuts, it said.

Royal Mail said there would be "fewer than 1,000" voluntary redundancies, but it expects no compulsory redundancies.