The price of posting standard first and second class letters has risen.
A first class stamp for smaller items weighing up to 100g now costs 36p, up 2p. The cost of a second class stamp has gone up from 24p to 27p.
Consumer watchdog Postwatch criticised the increases, announced in December, after Royal Mail's deliveries were hit by last year's industrial action.
Despite the rises, Royal Mail estimates it will still lose 6p on every stamped letter and parcel it delivers.
Last year, Royal Mail lost £178m on stamped mail.
It has also come under pressure from competitors in the more lucrative business mail market.
Losing business
Some large companies have switched their business to firms they consider to be either cheaper or more efficient than Royal Mail.
"For many years profits from business mail have funded stamped mail losses," Luisa Fulci, Royal Mail's director of commercial pricing and policy, said.
"But it's getting harder all the time to maintain that subsidy as competition for business mail from rivals, electronic communications and the wider communications market is increasing while mail volumes fall,” she added.
But consumer group Postwatch said Royal Mail should "make greater strides to improve efficiency rather than making its customers pick up the bill for management failure".
The cost of sending bigger letters has also risen. A large first class letter has gone up from 48p to 52p and a large second class letter up from 40p to 42p.
The cost of sending items by Special Delivery has increased, too. The fastest service has gone up by 95p to £10.30 for items up to 100g.
The "Next Day" Special Delivery service has risen 30p to £4.60.
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