RT said NatWest, owned by Royal Bank of Scotland Group (RBS), which is majority-controlled by the British government, had not explained why it was withdrawing its banking services in Britain and accused the bank of attacking freedom of speech.
In a letter published on RT's web site, NatWest said it had decided to withdraw its services after "careful consideration". It did not offer an explanation for what it said was a final ruling, but said it would cancel RT's services from Dec. 12.
In a statement later, RBS said: "These decisions are not taken lightly. We are reviewing the situation and are contacting the customer to discuss this further. The bank accounts remain open and are still operative."
Western critics dismiss RT, whose British arm produces UK-specific content, as a Kremlin mouthpiece designed to sow disinformation. The channel, previously known as Russia Today, says it offers a refreshing Russian-slanted alternative take on global events to mainstream Western media.
NatWest's decision is likely to inject further tension into relations between Moscow and London, already strained over Syria and Ukraine, and make it harder for RT to operate in Britain, something the channel said it would continue doing regardless.
"The situation will cause some insurmountable obstacles for the normal functioning of the channel in Britain," the Russian Foreign Ministry said in a statement, adding it was deeply concerned and would raise the matter with British authorities.
"This gives rise to the thought that the bank has taken the decision in agreement with official London to get rid of a news resource that is inconvenient for the official narrative but popular among the British public."
RT, which counts President Vladimir Putin among its supporters and relies on Russian state funds, said the bank's decision was consistent with "countless measures" taken in Britain and elsewhere in Europe to try to impede its work.
'A matter for the bank'
A spokeswoman for Prime Minister Theresa May distanced the British government from the row, saying it was up to NatWest who it offered services to based on its own appetite for risk.
Margarita Simonyan, editor-in-chief of RT, reacted sarcastically to the move on social media, writing "Long live freedom of speech!".
British broadcast regulator Ofcom has ruled against RT in the past, saying programs on Syria and Ukraine were misleading and flouted Britain's broadcasting code. RT has called those decisions unfair.
A similar row erupted last year when the Russian government accused Barclays of censorship after the British bank accounts of state news agency Rossiya Segodnya were shut. The agency's head, Dmitry Kiselyov, is on a European Union sanctions list related to Russia's actions in Ukraine.