The Prime Minister was admitted to a London hospital on Sunday evening – 10 days after testing positive for the virus – and stayed the night there.
A report by RIA Novosti then claimed the PM was on “artificial lung ventilation”, and had been rushed to hospital.
Part of Vladimir Putin’s state run media, the report has now been slapped down by No 10.
The PM’s spokesman said: “That is disinformation.
“Our specialist Government units have seen a rise in false and misleading narratives since the coronavirus pandemic began.
“It is important it is knocked down quickly.
“We are working closely with social media companies to stem the spread of falsehoods and rumours.”
The news agency claimed it’s source was “close to the top of England’s national healthcare system”, but was not named.
It was also shared by Sputnik, another state-owned new outlet said to be linked to the Kremlin.
They said: “UK Prime Minister Boris Johnson, who has tested positive for COVID-19, will get artificial lung ventilation, a source close to the UK healthcare authorities told Sputnik.”
The fake news was also bashed by BBC Home Affairs Correspondent Daniel Sandford, who used to work in Moscow for the corporation.
He tweeted: “Well, either the Russian news agency has amazing sources or this is a shocking lie.
“Assuming this is untrue, it is a truly dreadful bit of misinformation.”
It comes as The Times today claimed the PM was given oxygen treatment after arriving at the hospital.
No10 stressed it was not an emergency trip and was a planned visit on advice from his doctor.
The admission was a “precautionary step” as he continued to have “persistent symptoms”, thought to be a high temperature.
The move comes after aides became increasingly worried about his health as Mr Johnson continued to show symptoms more than a week after he tested positive.
Government aides said Boris had been “coughing and spluttering” on video conference calls over the past few days.