Britain drew the ire of Russia on Friday after it fined the Kremlin-backed broadcaster RT ?200,000 for "serious" bias in its coverage of last year's Salisbury nerve agent poisoning and the Syria conflict.
The Russian foreign ministry called it an "act of censorship" and warned British media operating in Moscow "to face the consequences".
RT had earlier denounced the penalty as "astonishing".
The international TV channel is already seeking a judicial review against the investigation at London's High Court.
The broadcaster rejects accusations of being a Russian state propaganda channel and says it presents an alternative -- and independent -- take on world news.
Britain's Ofcom media regulator said Friday that RT failed to follow UK impartiality laws in seven news and current affair programmes that aired between March 17 and April 26 of 2018.
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The ?200,000 (USD 250,000, 225,000 euro) fine was levied on RT's parent company TV-Novosti -- a non-profit organisation founded by Russia's RIA Novosti state news agency.
The regulator also required RT to broadcast a summary of the ruling to its viewers.
"Taken together, these breaches represent serious and repeated failures of compliance of our rules," Ofcom said in a statement.
"The programmes were mostly in relation to major matters of political controversy and current public policy -- namely the UK government's response to the events in Salsbury, and the Syrians conflict."
"It is astonishing that, in contrast, Ofcom sees RT's programmes... as worthy of greater sanction than programmes containing hate speech and incitement to violence."