Vladimir Putin said Boris Johnson’s “energy, optimism and sense of humour” would help him defeat coronavirus.
The Prime Minister was admitted to hospital on Sunday, before his condition worsened and he was moved to intensive care on Monday night.
Several world leaders have sent well wishes to Johnson, with Putin the latest to offer his support.
In a “telegram” to Mr Johnson, the Russian president said: “Dear Prime Minister,
“I would like to express my sincere support to you during this difficult time.
"I am certain that your energy, optimism and sense of humour will help to defeat the illness.
"I sincerely wish you a speedy and complete recovery.
"Respectfully yours,
“Vladimir Putin”
The message was sent the same time Putin's spokesman bizarrely claimed that Britain had not sought Moscow’s help to cure the prime minister.
He was cited by RIA Novosti as saying London “has not asked Russia for help in curing Johnson from coronavirus”.
British doctors “will be able to take care of him themselves”, said the report.
Meanwhile, Mr Johnson's condition is "stable", Downing Street said on Tuesday in an update after he was taken into intensive care on Monday night, according to sources.
Mr Johnson was given oxygen last night and taken to intensive care at central London's St Thomas' Hospital, in what Downing Street said was a precautionary measure.
Medical experts say the hospital has one of the best ICUs in the country, meaning the PM will be getting the best possible care while being treated there.
He has been give oxygen prior to going into intensive care, though has not been put on a ventilator and does not have pneumonia.
Uniformed police officers have been standing at the main entrance since the PM's arrival on Sunday and police vehicles have been seen in the area.
Mr Johnson's heavily pregnant fiancee Carrie Symonds is barred from visiting him as he battles the virus alone in intensive care.
The 32-year-old former political adviser is just weeks away from giving birth and previously revealed she was also struck down by the killer bug last week.
She is unable to see the PM due to her pregnancy and because visitors are banned in an effort to contain the spread.
