Britain's communications regulator has fined Royal Mail STG50 million ($A88 million) for a "serious breach" of competition law.
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Ofcom said the company abused its dominant position by discriminating against its only major competitor for delivering letters, Whistl.
The penalty follows an investigation into a complaint by Whistl, one of Royal Mail's wholesale customers.
The complaint was linked to changes Royal Mail made to its wholesale customers' contracts in 2014, including price increases.
The price rises meant that any of Royal Mail's wholesale customers seeking to compete with it by delivering letters in some parts of the UK, as Whistl was, would have to pay higher prices in the remaining areas - where it used Royal Mail for delivery.
Following notification of these new prices, Whistl suspended plans to extend delivery services.
Ofcom's investigation found Royal Mail's actions amounted to "anti-competitive discrimination against customers, such as Whistl, who sought to deliver bulk mail".
Jonathan Oxley, Ofcom's competition director, said: "Royal Mail broke the law by abusing its dominant position in bulk mail delivery.
"All companies must play by the rules. Royal Mail's behaviour was unacceptable, and it denied postal users the potential benefits that come from effective competition."
Royal Mail said it will appeal against Ofcom's decision, claiming its price changes were "never implemented or paid".
Australian Associated Press