The postal service said it has limited shops' supplies in a bid to prevent retailers from profiting at its expense from the hike in stamp prices that comes into force on April 30.
The Royal Mail wants instead to ensure that it benefits from the price rises, which will see the price of first-class stamps increasing by 30% from 46p to 60p from 30 April, and second-class stamps rising by 39% from 36p to 50p.
The postal service denied that the cap has led to a shortage of stamps, claiming that retailers had a "reasonable and proportionate allocation".
Royal Mail spokesman James Eadie said: "We do have a sensible allocation in place so that individual retailers can pre-order in advance of the price rise, based on their normal full-year expectations of demand. These allocations are in place for all retailers so that we can balance the customer demand with the need to protect Royal Mail's revenues.
"This is a prudent and appropriate policy. Our priority is to ensure that the proceeds from this much needed price rise go to sustain the six-day-a-week service, which has been loss making for some time.
"We have put arrangements in place with all our major retailers to ensure that they have adequate stocks of stamps to meet customer demand."
The spokesman added he was "fully satisfied" that post offices have enough stamps to meet customer demand.
"There is no shortage of stamps. Therefore, by definition, there is no rationing. Companies only ration if there is a genuine shortage, which there is not," he said.
Retailers said they had seen a fivefold rise in sales as people stocked up on stamps at the cheaper price, the Daily Telegraph reported.
The paper said that Ocado, the online supermarket, ran out of stamps entirely earlier this month, while Asda, Sainsbury's and Morrisons had seen a sharp rise in demand.
Ian Murray, the shadow postal affairs minster, told the paper he would write to Ofcom, the postal regulator, urging it to investigate the "shameless profiteering at the public's expense" by Royal Mail.
