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.
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”.
The price rises also come as many households face rising living costs, with food, energy and fuel costs all going up.