Ofcom has fined a Russian news service £200,000 for "a serious breach" of impartiality rules in several news and current affairs programmes.
The broadcasting regulator said RT's breaches included reports on the poisoning of Sergei and Yulia Skripal in Salisbury and the Syria conflict.
Ofcom has instructed RT, formerly Russia Today, to broadcast a summary of its findings.
The Kremlin-funded broadcaster called the fine "disproportionate".
The alleged breaches occurred in seven programmes broadcast between March and April 2018, an investigation by the regulator found in December.
The programmes were mostly in relation to major matters of political controversy including the Ukrainian Government's position on Nazism and its treatment of Roma Gypsies.
Two of the seven programmes featured former MP George Galloway.
Ofcom said: "RT's failings were a serious breach of our due impartiality rules, which protect public trust in news and other programmes," adding RT's breaches represented serious and repeated failures of compliance with its rules.
RT has denied the breaches and has launched legal proceeding against Ofcom's ruling. The watchdog said it would not enforce the fine until the proceedings have been concluded.
In a statement, RT said: "It is very wrong for Ofcom to have issued a sanction against RT on the basis of its breach findings that are currently under Judicial Review by the High Court in London.
"While we continue to contest the very legitimacy of the breach decisions themselves, we find the scale of proposed penalty to be particularly inappropriate and disproportionate per Ofcom's own track record."
RT claimed other breaches of the broadcasting code "involving hate speech and incitement to violence have been subject to substantially lower fines".