Royal Mail has been accused of being “shortsighted” and risking a drop in the number of charity Christmas cards sent this year as it increases stamp prices for the fifth time in less than three years.

The price of a first-class stamp will rise by 30p on Monday, to £1.65, far outstripping the rate of inflation. Second-class stamps will remain at 85p.

The increase adds to a 10p rise in the cost of first- and second-class stamps in April, which took them to £1.35 and 85p respectively.

The rise in stamp prices has triggered fears that the amount of money reaching charities through greetings cards will be curbed this Christmas.

Christine Ansell, the chief executive of the charity card specialist Cards for Good Causes, said: “We already know that the general public’s funds are vastly reduced due to the cost of living crisis, so making it more difficult to send greetings cards quickly and affordably will mean shoppers are less likely to spend with us this Christmas.”

Cards for Good Causes is a non-profit trading subsidiary of the 1959 Group of Charities, an organisation includes Cancer Research UK, Barnardo’s and the British Heart Foundation. Its cards are sold online and at Christmas pop-up stores, and it has raised £22m for charities over the last decade.

The price increase will have a “direct impact” on the funds Ansell’s charity is able to raise, and take an emotional toll on the recipients of greetings cards – many of whom are elderly and socially isolated, she said.

She added: “Posties are unsung heroes in a society like ours where loneliness is on the rise, and receiving a handwritten card can be the equivalent of a hug for those experiencing social isolation.

“Royal Mail is being very shortsighted – an affordable, reliable postal service is essential for the UK’s financial and emotional wellbeing, and a lifeline for the socially isolated.”

Last month, Citizens Advice called on the communications regulator, Ofcom, to hold Royal Mail to account and stop “letting the company get away with setting rocketing prices in the wake of half a decade of missed delivery targets”.

Royal Mail has been missing delivery targets and is lobbying Ofcom to be allowed to reduce second-class deliveries to alternate weekdays to save costs.

Nick Landon, the chief commercial officer at Royal Mail, said: “We always consider price increases very carefully. However, when letter volumes have declined by two-thirds since their peak, the cost of delivering each letter inevitably increases.

“A complex and extensive network is needed to get every letter and parcel across the country for a single price – travelling on trucks, planes, ferries and in some cases drones before it reaches its final destination on foot.”

Royal Mail’s parent company, International Distribution Services, is the subject of a £3.57bn takeover offer from the Czech billionaire, Daniel Křetínský. The government is scrutinising the energy tycoon’s bid for any national security risks.