First class stamp up 5p to 46p
The price of a first class stamp has risen by 5p to 46p and by 4p to 36p for second class post, with the Royal Mail insisting they are still among the lowest in Europe.
Consumer Focus said customers would be "extremely disappointed" by the latest increases, pointing out that stamp prices have gone up at double the rate of inflation over the last five years.
Deputy chief executive Philip Cullum said: "Royal Mail needs to modernise but customers are being asked to pick up the tab. In return, they will expect to see a far more efficient, effective and competitive service. Consumers can't be expected to continually bail out an inefficient Royal Mail."
A Royal Mail spokesman said: "After these increases, Royal Mail will continue providing value-for-money as our prices will still be among the lowest in Europe.
"We are investing heavily to modernise our operations, which is all about providing our customers with the services they need in today's open, highly competitive postal marketplace.
"With the sharp declines in mail volume - a 20% fall in five years - our revenues are falling. That means if we don't generate more income, we will simply not be able to keep funding our six-days-a-week collection, sorting, transport and delivery operation to the UK's 28 million homes and businesses.
"Royal Mail has very significant fixed costs as its postmen and women collect and deliver the mail in every community nationwide, six days a week. Last financial year, Royal Mail lost £163 million on stamped mail. That was equivalent to a 6.4p loss on average on every stamped item we handled."