The price of a first-class stamp has jumped by 10p to £1.80, effective from Tuesday.
It comes despite Royal Mail's persistent failure to meet delivery targets and growing concerns over its service performance.
The cost of a second-class stamp has also increased by 4p to 91p. However, stamps purchased prior to the changes will remain valid for use.
This marks the eighth increase for first-class stamps in six years, seeing their price more than double by 137 per cent.
The price of second-class stamps has also risen six times in the same period.
This latest hike follows Royal Mail’s February announcement that it had again missed delivery targets in the most recent quarter.
Announcing the decision in March, Royal Mail said the price change reflected the continued rise in the cost of delivery as letter volumes fell and the number of addresses surged.
Richard Travers, managing director of letters at Royal Mail, said: “We always consider price changes very carefully, balancing affordability with the rising cost of delivering mail.
“On average, UK adults now spend just £6.50 each year on stamps and there are 70 per cent fewer letters sent than 20 years ago.
“In the meantime, the number of addresses we deliver to has increased by four million to 32 million addresses across the UK.”
The last time Royal Mail met its annual target for delivering first-class post on time was in 2019/20.
The firm – whose owner International Distribution Services was bought last June for £3.6bn by Czech billionaire Daniel Kretinsky’s EP Group – repeated its call to “urgently move forward” with reforms to the service.
Anne Pardoe, head of policy at Citizens Advice, earlier said: “More than half a decade has gone by since the company met its delivery targets and people still face a gamble, with many uncertain if their important documents or letters like medical appointments will arrive on time.
“Things only risk getting worse when cuts to delivery days and reduced performance targets come into full effect.
“Against this backdrop, Ofcom simply cannot wave through these increases any longer. Higher prices must come with higher standards – increases should be tied to Royal Mail’s performance on the doorstep.”
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