The price of a first class stamp has risen by 10p to 95p as Royal Mail seeks to cover higher delivery costs while the number of letters being sent falls.
Second class stamps have also increased by 2p to 68p.
Royal Mail said it was having to deliver to a growing number of addresses, which was adding to costs.
But some people criticised the move at a time when the cost of living is increasing and the firm's profits have jumped.
The price of first and second class stamps have more than doubled since 2010, when they cost 41p and 32p respectively. In 2000, it was 27p and 19p.
Last year, Royal Mail saw the volume of parcels it delivers surge as people embraced online shopping, more than offsetting its losses from letters.
Its annual profits quadrupled to £726m in the year to 28 March - up from £180m a year earlier.
Unite general secretary Sharon Graham accused Royal Mail's boardroom of "again raising prices while helping itself to massive profits".
She said: "It is behaving like a short-term greedy speculator rather than the responsible owner of a key UK public service."
Last month, Royal Mail's chief operating officer Nick Landon said the price changes were necessary to ensure the business remained "sustainable" and "relevant".
He said the volume of letters the firm delivers is down more than 60% since its peak in 2004-05, while the number of addresses it delivers to has grown by around 3.5 million, driving up its costs.
"We need to carefully balance our pricing against declining letter volumes and increasing costs of delivering to a growing number of addresses six days a week," he said.
The trend forced the company into a painful turnaround plan in 2019, although its fortunes have changed during the pandemic.
The price rises also come as many households face rising living costs, with food, energy and fuel costs all going up.
One person tweeted: "The increases seem more and more frequent (like just about everything else these days!) I will definitely be buying less [stamps] now - which is a shame, as I love sending cards etc to my loved ones."
Another tweeted: "I know inflation is to blame for the price, but why do we always pay more for less service?"
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It comes as Royal Mail, whose deliveries were hit by severe delays in 2021, faces the prospect of strikes over plans to cut managers' jobs.
The union Unite last week said the company aims to sack nearly 1,000 managers and bring in lower rates of pay in another case of "fire and rehire", which Royal Mail denies.
The union is preparing an industrial action ballot, which it said could see strikes begin in April.
Unite said there was "no excuse" for the cuts at a time when the company was growing strongly. But a spokesman for Royal Mail said the restructure would improve services.
"The proposals are designed to simplify and streamline our operational structures," he said. "We firmly believe this change will deliver a number of benefits for our managers and our customers and will be a significant step forward in the reinvention of Royal Mail."
Source: BBC News