Older people will be most affected by the stamp increases, warned Age Concern Slough and Berkshire East.
First class stamps will rise from 46p to 60p from Monday, April 30, while a second class stamp will rise from 36p to 50p in the biggest annual percentage increase since 1975.
Age Concern Slough and Berkshire East chief executive Tracey Morgan (pictured) said: "The truth is that letters are for people who cannot send emails.
"Older people are being hit in their pockets yet again, at a time where they simply cannot increase their income and in many cases are not able to get online or text."
She also said the increase will affect the charity which has been hit by funding cuts.
"My own charity will be severely affected since many of our volunteers and contacts are with older people, who have no email access and so send out many letters, every day."
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