The Royal Mail is increasing the cost of postage, with first-class stamps rising 6p to 76p and second-class up 4p to 65p.
Prices will climb from 23 March, less than a year after they were hiked to their current levels.
The 65p second-class stamp is the maximum under an Ofcom price cap.
“These changes are necessary to help ensure the sustainability of the one-price-goes-anywhere Universal Service,” Royal Mail said.
It blamed the increases on “a challenging business environment”.
Stephen Agar, managing director of letters at Royal Mail said: “We are operating in a tough market at present, under the threat of making a loss by 2021. These price increases will help us maintain the quality of service that is expected by our customers,”
The likelihood has increased that Royal Mail in the UK will be loss-making in 2020-21, the company said in a statement, adding: “We want to invest £1.8bn in the UK to turnaround, grow and sustain the Universal Service.”
It said it understands that many companies and households are finding it hard in the current economic environment.
“As a result, we have considered any pricing changes very carefully and in doing so have sought to minimise any impact on our customers.”