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 programs that aired between March 17 and April 26 of 2018.
The £200,000 ($250,000, 225,000 euro) fine was levied on RT's parent company TV-Novosti -- a non-profit organization 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 programs 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 -- available in English in the United States and Europe as well as Arabic in the Middle East -- immediately appealed.
- Tit-for-tat probes -
The Russian media regulator responded to December's ruling by opening an investigation into the BBC for allegedly promoting the material of a "terrorist group" through its Syria coverage.
The BBC probe was announced within hours of the original Ofcom decision and remains open to this day.
RT said it was "very wrong" for Ofcom to issue the fine while its appeal was under judicial review.
A spokesman told AFP that RT contests "the very legitimacy" of the December breach decision -- as well as the size of Friday'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 programs... as worthy of greater sanction than programs containing hate speech and incitement to violence."
The foreign ministry statement further accused UK authorities of "trying to limit the activity of Russian media in the country".
RT does not have to pay the fine or broadcast Ofcom's decision until the High Court completes its review over the coming months.
- Salisbury and Douma -
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 runup to the 2018 Russia World Cup.
Moscow 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 programs "focused overwhelmingly on expressing doubt over the UK government's position that the Russian government bore any responsibility for the (Salisbury) incident".
Ofcom said RT failed to give "due weight" to "an appropriately wide range of significant views".
The program 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.
AFP/File / ALFREDO ESTRELLA 
AFP/File / ALFREDO ESTRELLA 
AFP/File / CARLOS OGAZ 
AFP/File / Angela Weiss 
TT NEWS AGENCY/AFP/File / Fredrik PERSSON