The cost of a first-class stamp will rise by 5p to £1.70 on 7 April, Royal Mail has announced.
It will also raise the price of a second-class stamp by 2p to 87p on the same day.
The company said the decision carefully considered “balancing affordability with the increasing cost of delivering mail”.
Citizens Advice described the change as “yet another blow to consumers”, and said the change to the second class price was “unjust”.
The number of letters Royal Mail delivers has fallen from a peak of 20 billion in 2004-05 to 6.6 billion last year.
However, the price of stamps has continued to rise. Since 2022, Royal Mail has already hiked the cost of a first-class stamp five times from 85p to £1.65.
The next increase, in April, was the result – it said – of delivering fewer letters to more addresses.
“We always consider price changes very carefully but the cost of delivering mail continues to increase,” said Nick Landon, chief commercial officer at Royal Mail.
“A complex and extensive network of trucks, planes and 85,000 posties is needed to ensure we can deliver across the country for just 87p.”
But consumer advocate Citizens Advice said millions of people would be forced to pay more while also suffering from postal delays.
“It’s unjust for Royal Mail to raise the price of a second-class stamp, while the regulator Ofcom looks at reducing second-class deliveries to alternate weekdays,” said Tom MacInnes, the charity’s director of policy.
“As first-class stamps are becoming unaffordable, people could be forced by price pressures into choosing a slower service.”
In January, regulator Ofcom proposed that Royal Mail should only deliver second-class letters every other weekday and not on Saturdays to protect the future of the UK’s postal industry.
The one-price-goes-anywhere Universal Service Obligation (USO) means Royal Mail has to deliver post six days a week, from Monday to Saturday, and parcels on five from Monday to Friday.
Ofcom has launched a consultation on the new proposals, which is open to the public until 10 April. A decision is expected in the summer.
Royal Mail’s parent company is being sold to a business controlled by Czech billionaire Daniel Kretinsky in a deal worth £3.6bn, after the Labour government approved the deal last year.
The government will maintain a “golden share” which means Mr Kretinsky’s business will have to get approval for any changes to Royal Mail’s ownership, the location of its headquarters and its tax residency.
Royal Mail must also adhere to the USO, which Mr Kretinsky has pledged he will do for “as long as I am alive”.
