The Royal Mail has capped the number of stamps shops can buy to stop customers stockpiling ahead of price rises later this month.
From April 30 the cost of a first-class stamp will increase from 46p to 60p, while a second class one will go up from 36p to 50p.
The postal service has limited supplies to 20% of each retailer's annual allocation in a bid to "protect revenue" and prevent shops cashing in on the higher prices.
Royal Mail spokesman James Eadie said the revenue from the increase was needed to maintain the six-day a week postal service, which has been loss-making for some time.
He 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 we can balance the customer demand with the need to protect Royal Mail's revenues.
"This is a prudent and appropriate policy."
The National Federation of SubPostmasters defended the move by Royal Mail.
Royal Mail is limiting the number of stamps on sale before the price hike
The union's general secretary George Thomson told Sky News that the postal service was trying to ensure there was an even spread of stamps available across the country, as well as stop individual suppliers profiteering.
Mr Thomson admitted their had been "a run" on stamp supplies ahead of the price hike.
"At some of the post offices it's like Christmas," he said.
Royal Mail refused to comment on reports of individual shops running out of supplies but said: "Individual retail chains are given a reasonable and proportionate allocation of stamps in advance of any price rise.
"We will continue to discuss stamp supplies with any retail customer which feels it requires more to meet any real customer demand."
Demand for stamps surged when the price increase was announced last month but has since fallen back, Royal Mail said.
Mr Eadie said: "There is a good supply of stamps across the country. We have more than adequate stock in place to meet customer demand.
"Stamps are available from a very large number of outlets across the UK - approximately 45,000 in total."
Labour's shadow postal affairs minister Ian Murray told the Telegraph newspaper: "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."
When the prices increases were announced in March consumer groups warned the postal service would come under greater scrutiny from its users.
Robert Hammond from Consumer Focus said: "This is not great news for consumers.
"This is a very significant increase in the price of an essential service and those consumers who continue to use it will look much harder at the value for money and quality of service that they get."