Russia has threatened Moscow-based British media with retaliation after the UK's media watchdog imposed a heavy fine on Russia's state-funded RT broadcaster.

Britain's media watchdog has fined Russia's state-funded RT broadcaster £200,000 (€225,000) for "serious failures" in its coverage of last year's Salisbury nerve agent poisoning and the Syria conflict.

"British media working in Russia should be prepared to face the consequences of London's actions," the Russian foreign ministry said in a statement.

The fine is an "act of censorship", it added.

Russia said the fine "far exceeded" those imposed on other media, adding RT had been unfairly singled out.

"Once again we are witnessing how British authorities are trying to limit the activity of Russian media in the country," it said, claiming that this was part of an "anti-Russian campaign" in Britain.

In the statement, the ministry also accused UK media of "regularly" misreporting news on Russia.

RT immediately denounced the penalty as "astonishing".

The international broadcaster is seeking a judicial review against the investigation at London's High Court.

Ofcom said RT failed to follow Britain's impartiality laws in seven news and current affair programmes that aired between 17 March and 26 April of 2018.

The fine was levied on RT's parent company TV-Novosti - a non-profit organisation founded by Russia's RIA Novosti state news agency.

"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."

Ofcom initially found RT in breach of Britain's strict media rules last December.

The global broadcaster, which is available in English in the United States and Europe, as well as Arabic in the Middle East, immediately appealed.

The Russian broadcaster said it was "very wrong" for Ofcom to issue the fine while the case was still under judicial review.

A spokesman told AFP that RT contests "the very legitimacy" of the December breach decision, as well as the size of today's penalty.

"Cases that involved hate speech and incitement to violence have been subject to substantially lower fines," the RT spokesman said.

"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."

The poisoning of former Russian spy Sergei Skripal and his daughter Yulia with a Soviet-era nerve agent in the English city of Salisbury in March 2018 prompted London to sever diplomatic relations with Moscow.

It also set off the largest-ever wave of diplomatic expulsions between Moscow and Western allies in the run-up to the 2018 Russia World Cup.

Russia denies involvement despite CCTV footage capturing two men UK authorities identify as Russian military intelligence officers walking near Skripal's home on the day of the attack.

The Skripals fell into comas that lasted days and have since gone into hiding.

The December judgement found that one of RT's programmes "focused overwhelmingly on expressing doubt over the UK government's position that the Russian government bore any responsibility for the (Salisbury) incident".

Ofcom ruled that RT failed to give "due weight" to "an appropriately wide range of significant views".

The programme on Syria disputed President Bashar al-Assad soldiers' responsibility for the April 2018 Douma gas attack in which the OPCW global chemical weapons watchdog said more than 40 died.

The Russian foreign ministry and the Kremlin issued no immediate response to the fine. The RT spokesman said the broadcaster was "duly considering further legal options".