The Royal Mail has imposed a cap on the number of stamps available to buy ahead of a significant hike in price.
Shops will only be allowed to purchase up to 20% of their annual allowance of stamps before the price rise comes into effect on 30 April, leaving some fearing that shops will run out.
The shadow postal affairs minister, Ian Murray, told the Telegraph: "The disproportionate rise in the cost of stamps will have a significant impact on small businesses and those on low incomes.
"It's understandable that those who will bear the brunt of the increase will want to stock up before the price increase."
However, Royal Mail spokesman Jamie Eadie denied this was the case: "Individual retail chains are given a reasonable and proportionate allocation of stamps in advance of any price rise.
"How they then decide to use that allocation is a matter for them, particularly if they choose to discount those stamps as part of their overall retail proposition."
The demand for stamps has risen sharply since the Royal Mail announced a 14p rise in the price of both first class and second class stamps. A first class stamp will now cost 60p, up from 46p, with second class stamps costing 50p.
Shops have reportedly been stockpiling stamps in an attempt to get the most of the current price, which the Royal Mail believes is against the spirit of their objective.
Royal Mail's position is that shops shouldn't be allowed to buy up vast amounts of stamps before the new price rises, which would effectively lead to Royal Mail not getting the revenue increase it says it needs in order to "sustain the six-day-a-week service which has been loss making for some time."