Royal Mail customers hoping to exchange their old stamps have expressed their anger towards the Post Office after allegedly being told these are fake.

This comes amid a move by the national postal service to modernise its stamp system on Tuesday with customers encouraged to swap their old stamps for newer ones with barcodes.

However, the Royal Mail's free 'Swap Out' scheme has apparently run into roadblocks as reports claim some customers are unable to do so.

The Daily Telegraph reported that there had been some 40 examples of customers unable to exchange their stamps due to these being 'invalid'.

It revealed one example of a customer called Susan Harrison in Hartlepool, who sent out eight old stamps she had bought from a Post Office desk

However, she was unable to exchange these and received a letter informing her that the stamps were "not genuine".

She told the newspaper: "I'm gobsmacked. If you can't buy stamps in a post office without them being counterfeit then where can you buy them?"

Royal Mail should 'investigate' fake stamp issue 'as a priority' as Post Office issues statement

Of the reported issue, Which? spokesperson Lisa Webb told the MailOnline: "It's in Royal Mail's interests to investigate as a priority any issues reported with its new stamp barcode system.

"Anyone who believes they have been wrongly charged for stamps marked as counterfeit should raise it with Royal Mail and the company must ensure that any customers who encounter problems with the new system are not left out of pocket."

A Post Office spokesperson told the MailOnline: "Stamps are available to buy from a number of different sources. 

Post Office Ltd receives its stamps direct from Royal Mail's secure printers and are shared with our experienced Postmasters and operators to sell in their Post Offices.

"Customers who buy stamps at Post Offices are given an itemised receipt. 

"We take any allegation of fake stamps at our branches seriously and will always require a receipt as proof of purchase before we can investigate. 

"As part of this, any alleged fake stamp needs to be double checked by Royal Mail to verify the status of the stamp."

A Royal Mail spokesperson added: "We encourage any customer who believes they have been incorrectly surcharged to send the stamps they have remaining to us. 

"It is important that we can investigate and determine whether the stamps are genuine, as well as understand exactly where they were purchased."