According to the channel, NatWest bank said the decision "cannot be reviewed" after saying it will no longer "provide facilities" to the Russian state-owned broadcaster.
The Kremlin-funded television network Russia Today said on Monday that the United Kingdom's NatWest bank has closed its accounts in a unilateral decision.
Ties between Russia and the UK are at their lowest since the Cold War over the crisis in Ukraine and Moscow's bombing campaign in Syria.
There was no immediate indication that this decision was linked to the British authorities.
"They have closed our accounts in Britain. All the accounts," wrote RT editor-in-chief Margarita Simonyan on Twitter.
She said the channel had been told the decision "cannot be reviewed," adding "Long live freedom of speech."
. . ' '. !— (@M_Simonyan) October 17, 2016
RT later published on its website a letter from NatWest bank dated October 12th saying it has reviewed its banking arrangements with RT and reached the conclusion that they will no longer provide it facilities.
The letter from NatWest, owned by the Royal Bank of Scotland, does not give any reason for this decision, which it says is "final" and not open to any discussion.
The bank says it will close RT's accounts by December 12th.
The bank decision came as ongoing legal action involving claims against Moscow from shareholders of defunct oil firm Yukos, which was founded by Mikhail Khodorkovsky, has led to the freezing of Russian assets abroad, including those of state media.
RT reported on its channel that the letter was sent to RT's London office and that RT has been with NatWest for more than 10 years, calling the letter "very vague, very British."
Simonyan told the RBK business news site that she had "no idea" of the reason for the closure of the accounts, but suggested it could be linked to the fact that "we are expecting new British and US sanctions against Russia."
"Possibly it's linked to this," Simonyan said.
She told Rossiya-24 state television that "there can be no reason, we have absolutely transparent work, absolutely transparent financing."
She said the decision included some personal accounts of senior staff working in Britain.
RT was set up to present news from the perspective of the Kremlin, and has been criticised as a slavish propaganda tool for President Vladimir Putin.
Russian foreign ministry spokeswoman Maria Zakharova tied the bank's decision to the Brexit, writing on Facebook: "It looks like when leaving the European Union, London left in Europe all its obligations on freedom of speech."
The foreign-language network that is primarily aimed at audiences in Europe and the United States broadcasts 24-hours a day in English, Arabic and Spanish.
It has a British outlet called RT UK that broadcasts from London.