Royal Mail (LON: RMG) has been fined a record £50 million for breaching competition laws.

Ofcom, the communications watchdog, handed out its largest-ever fine on Tuesday following an investigation into a complaint made by Whistl over the increases to wholesale prices.

Ofgem said that the group’s actions “amounted to anti-competitive discrimination against customers, such as Whistl, who sought to deliver bulk mail”.

Ofcom’s Competition Group Director, Jonathan Oxley, 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,” he added.

Royal Mail plans to challenge the fine, saying that it “strongly refutes that it has acted in breach of the Competition Act”.

“For an allegation of abusive price discrimination to be established, the law is very clear.  The relevant prices must be actually paid. And, the party paying such prices must be placed at a competitive disadvantage as a result. In this case neither of these essential elements exist,” the group added.

Whistl has said that it may seek compensation from the firm.

Royal Mail received a fine in April this year £12,000 for sending 327,000 nuisance emails to customers who opted out of receiving the emails.

“We take the privacy of our customers extremely seriously,” said the group.

“We are very sorry that we let some of our customers down on this occasion. Following this incident, we have tightened up our processes and governance measures still further.”

Royal Mail’s former boss, Moya Greene, stepped down in June.