Boris Johnson chairs the morning Covid-19 Meeting remotely after testing positive. AP

British Prime Minister Boris Johnson, who was diagnosed with coronavirus late last month, was on Monday moved to an intensive care unit after his condition worsened, his office said.

The 55-year-old, who was admitted to a London hospital on Sunday evening for tests after continuing to suffer a cough and high temperature, has asked Foreign Secretary Dominic Raab "to deputise for him where necessary", the spokesman said.

Below is early reaction to the news:

BUCKINGHAM PALACE

The palace said Queen Elizabeth had been kept informed by Downing Street.

EMMANUEL MACRON

"All my support for Boris Johnson, his family and the British people at this difficult time. I wish him to overcome this ordeal quickly."

IVANKA TRUMP

"My thoughts and prayers are with Boris Johnson and his family. Godspeed Mr Prime Minister!"

JUSTIN TRUDEAU

"Sending my best wishes to Prime Minister Boris Johnson for a full and speedy recovery. My thoughts are with you and your family right now. Hope to see you back at Number 10 soon."

KEIR STARMER

"Terribly sad news. All the country's thoughts are with the Prime Minister and his family during this incredibly difficult time."

RISHI SUNAK

"My thoughts tonight are with Boris Johnson and (his fiancée) Carrie Symonds. I know he'll be getting the best care possible and will come out of this even stronger."

DAVID CAMERON

"Thinking of Boris Johnson and his family tonight. Get well soon. You are in great hands and we all want you safe, well and back in 10 Downing Street."

THERESA MAY

"My thoughts and prayers are with Boris Johnson and his family as he continues to receive treatment in hospital."

Johnson, 55, announced he had mild symptoms of COVID-19 on March 27 and had been in self-isolation at his Downing Street residence for seven days.

He had been scheduled to re-emerge on Friday after a week of recovery and working remotely, but said he would remain at home because he still had a high temperature — one of the symptoms.

Downing Street said in a statement on Sunday: "On the advice of his doctor, the Prime Minister has tonight been admitted to hospital for tests.

"This is a precautionary step, as the Prime Minister continues to have persistent symptoms of coronavirus 10 days after testing positive for the virus."

It is understood Johnson's admission to hospital was not an emergency and was considered sensible because of his ongoing symptoms.

Johnson is the most high-profile world leader to contract the virus. His pregnant partner, Carrie Symonds, said she has been ill with symptoms for a week but was now recovering.

Boris Johnson washes his hands as he visits a Laboratory in Bedford technology Park, England. File/ AP

Health Secretary Matt Hancock returned to work on Friday after a week at home following his positive test for COVID-19. The chief medical officer, Chris Whitty, has also displayed symptoms.

Johnson's close adviser Dominic Cummings has also had to self-isolate.

Downing Street said at the time of Johnson's announcement that Foreign Secretary Dominic Raab would take charge if the prime minister was incapacitated.

But Johnson is understood to be still in charge of the government, and in contact with ministers and officials.

The former London mayor was seen on Thursday night, joining in a weekly round of applause in appreciation of healthcare workers tackling the outbreak.

In his video message on Friday, he said: "In my own case, although I'm feeling better and I have done my seven days of isolation, alas, I still have one of the minor symptoms... a temperature.

"So, in accordance with government advice, I must continue my self-isolation until that symptom itself goes."

Asked about the prime minister's health on Sunday, Hancock told Sky News television: "He's OK. I've been talking to him every day, several times a day throughout the time both of were off.

"He's very much got his hand on the tiller."

Agence France-Presse