People are unknowingly buying fakes from Chinese firms through online marketplaces, reports suggest.
Reports suggest a rise in complaints that stamps bought from legitimate stores are being deemed counterfeit. Anyone who receives a letter with a fake stamp is charged £5 by Royal Mail.
Conservative MP Iain Duncan Smith told BBC Breakfast: "China is behind it."
A Royal Mail spokesman said: "We are working hard to remove counterfeit stamps from circulation."
Consumers are being warned to look out for strange perforations around the edge of a stamp, a shine to the surface or the colour looking off.
Stamps no longer have a face value. They are 1st or second class.
As they put up the price each year it's becoming common to buy stamps before the price rise and sell them after.
The margin on the last rise was ~13% on 2nd class stamps, 8% on first class stamps.
13% has been roughly the average every year since 2005.
So you can absolutely buy stamps at less than "face value". Someone who bought them 4 years ago could easily give you a 20% discount and still make a profit.
As stamps are not allowed to expire (or have to be replaced if they do) this is a safe investment.
Royal mail have encouraged this to inflate sales in the short term and are suffering from those valid stamps still being available now with no further revenue.
Taking the face value off stamps is what's caused this problem.
There was never an investment opportunity in buying a 90p stamp that was still worth 90p postage years later.
But buying 1000 2nd class stamps that are always worth 2nd class postage has been an inflation beating purchase.
It would also negate the point of the legislation that means they have to accept stamps in the first place.
You should not have to visit a post office in person or online to post a letter.
There are letter boxes in walking distance. If you've bought a book of stamps everything you need is in your desk.
That's the system we have and it would never be designed by a business that way. But it's a business that's taken on that system alongside the I infrastructure for it.
If you genuinely depend on the post accessibility to it is important. It could be modernised but it was working before, modernisation and cost saving are not the same thing.
As it says in the article, these are also being purchased by small retailers, so even if you buy from a bricks-and-mortar store, there's no guarantee that you're getting the genuine article.