The price of a first class stamp is to rise above £1 in April, Royal Mail has announced.
From 3 April, first class stamps will increase by 15p to £1.10, while second class stamps will rise by 7p to 75p.
Royal Mail said the increases were needed to ensure the “one-price-goes-anywhere Universal Service remains sustainable”.
But Citizens Advice said nobody should be paying more for a “subpar service” and called for the regulator to act.
The price increases will come in the day before the first stamps with the image of King Charles go on general sale.
Royal Mail said the changes had been subject to “careful consideration” with letter volumes down 25% since the pandemic and with the business facing rising costs.
“We appreciate that many businesses and households are facing a challenging economic environment and we are committed to keeping our prices affordable,” said Nick Landon, chief commercial officer at Royal Mail.
“We have to carefully balance our pricing against a continued decline in letter volumes and the increasing costs of delivering letters six days a week to an ever-growing number of addresses across the country.”
However, the move was sharply criticised by Citizens Advice.
“These record-breaking prices couldn’t be coming at a worse time for consumers, who’ll now be paying 64% more for a first class stamp than five years ago,” said Matthew Upton, director of policy at Citizens Advice.
He added that with millions missing important letters as result of postal delays, “nobody should be paying more for this kind of subpar service”.
“Ofcom should be holding Royal Mail to account, but it’s letting the company get away with rocketing prices and over two years of missed delivery targets.”
The regulator said it caps the price of second class stamps “to make sure an affordable option is always available”.
But it said Royal Mail needed flexibility on pricing first class stamps “to make sure the universal postal service can continue”.
Royal Mail said it remained “committed” to the Universal Service, which means it is required to deliver letters at a uniform price to UK addresses six days a week.
However, it added that the costs of meeting it were increasing, with the number of letters sent having fallen from more than 20 billion in 2004-05, to about eight billion a year now.
It added that it expected to report a full-year loss of between £350m and £450m.
Last year, Royal Mail asked the government if it could drop Saturday letter deliveries, by changing its Universal Service Obligation from six days a week to five.
Royal Mail has also been hit by strike action in a long-running dispute with the Communication Workers Union over pay and conditions.
The company has said that the strikes, which included several in the run-up to Christmas, have cost it millions.