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The price of a first class stamp has risen by 10p to 95p this Monday and second class stamps have increased by 2p to 68p.
Royal Mail (RMG.L) said it was having to deliver to a growing number of addresses, which was adding to costs.
Letter volumes have fallen by more than 60% since their peak in 2004/5 and by around 20% since the start of the pandemic.
However, Royal Mail saw its annual profits quadruple to £726m ($953m) as the volume of parcels delivered surged during the pandemic.
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."
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Union Unite has accused Royal Mail of plans to sack nearly 1,000 managers and bring in lower rates of pay in another case of "fire and rehire", which the company has denied.
“With plans to slash 900 postal manager jobs and threats issued to Unite that collective bargaining agreements for our members will be ignored, Royal Mail’s owners are ruining this essential service. Ofgem has to get a grip because the universal service obligation is at serious risk,” Graham added.
Last month, Royal Mail's chief operating officer Nick Landon said the price changes were necessary.
“We understand that many companies and households are finding it hard in the current economic environment, and we will always keep our prices as affordable as possible,” he said.
“As customer needs change and we see a greater shift from letters to parcels, it is vital that the universal service adapts to stay relevant and sustainable.
“These price changes are necessary to ensure we can continue to maintain and invest in the one-price-goes-anywhere universal service for future generations.”