The price of a first class stamp has increased for the third time in 18 months.
This time, the cost has gone up from £1.10 to an eye watering £1.25.
Royal Mail has blamed the tough economic climate and increasing cost pressures, as reported by the BBC.
The company also cited its Universal Service Obligation (USO), which requires it to deliver letters to all 32 million UK addresses six days a week, as a key factor affecting the price rise.
Royal Mail wants to reform the USO, saying it is unsustainable, as the number of letters being sent is falling, while the number of households is growing.
There were 20 billion letters sent in 2004-05, compared to seven billion in 2022-23. The number of addresses has risen by four million over the same period of time.
Charity Citizens Advice has called on regulator Ofcom to hold the firm to account over the rising prices, while households struggle with the cost-of-living crisis. But the regulator said pricing flexibility was necessary for the postal service to remain viable.
Ofcom said it caps the price of a second class stamp, to make sure an affordable option is available, particularly for those on lower incomes.
The price of a second class stamp has remained at 75p.
The price of a first class stamp increased by 10p to 95p in April 2022. It rose to £1.10 in April this year.