British Prime Minister Boris Johnson on Wednesday became a father again when his partner Carrie Symonds gave birth to a baby boy, just weeks after he himself was hospitalised with coronavirus.

The news came as a surprise, as Symonds, 32, was not thought to be due for several weeks, but both she and the baby were said to be doing "very well".

The couple were recently confronted with the realities of the COVID-19 pandemic up close, after both contracted the potentially deadly virus.

Johnson, 55, only returned to work on Monday after being hospitalised, including three nights in intensive care, which he later said "could have gone either way".

Symonds, a former head of communications for the Conservative party, also reported having symptoms, although she recovered at home.

"The prime minister and Ms Symonds are thrilled to announce the birth of a healthy baby boy at a London hospital earlier this morning," a spokesman for the couple said.

"Both mother and baby are doing very well."

Johnson was present for the birth at an unnamed state-run National Health Service (NHS) hospital in London, according to his political spokesman.

He told reporters the Conservative leader had returned to work in Downing Street by Wednesday lunchtime, but that he would be taking some paternity leave.

"Later in the year, he'll take a short period of paternity leave," the spokesman said.

Messages of support for the pair poured in from around the world and across the British political spectrum.

Addressing his "dear friend" on Twitter, Japanese Prime Minister Shinzo Abe sent Johnson his "warmest congratulations on the birth of a sweet baby boy".

Australian Prime Minister Scott Morrison called it "an absolute blessing", noting the "tough time" Johnson had recently endured as he sent "best wishes from Australia".

In Britain, opposition Labour leader Keir Starmer said he hoped the birth would bring the couple "incredible relief and joy", while Scotland's First Minister Nicola Sturgeon wished "health and happiness to the wee one".

Johnson has been publicly dating Symonds since early 2019
Johnson has been publicly dating Symonds since early 2019 POOL / Jeremy Selwyn

Johnson has at least five other children, including four with his second wife, Marina Wheeler, from whom he split in 2018.

He also had a daughter as a result of an extra-marital affair while he was mayor of London, according to a 2013 court case.

Johnson and Symonds, who announced in February that they were engaged, last year became the first unmarried couple to live in Downing Street.

However, the new baby will not be the first -- former prime ministers Tony Blair and David Cameron both became fathers while in office.

Johnson's spokesman said the family planned to continue living in a flat above his Downing Street offices.

Johnson is the most high-profile world leader to contract COVID-19, and Britain is among the worst hit countries in the global pandemic, with more than 21,000 deaths so far.

The prime minister, who is under pressure to ease lockdown restrictions, spent two weeks recovering from coronavirus at his countryside retreat of Chequers, before returning to work on Monday.

There had been some speculation about his health after aides refused to say if he would attend the weekly prime minister's questions in parliament on Wednesday.

Foreign Secretary Dominic Raab, who deputised for Johnson during his illness, stepped in again.

However, Johnson held a lunchtime telephone call with Starmer to discuss the government's ongoing response to the coronavirus crisis.

Symonds worked on Johnson's re-election campaign when he was mayor of London, and later rose to become communications chief at the Conservatives.

She left to work for an ocean conservation charity, and uses her Twitter account to speak out against animal cruelty and plastic pollution.

Since the couple began publicly dating in early 2019, she has kept a low profile.

There was a brief controversy when, during Johnson's campaign for the Conservative leadership last year, police were called to the couple's south London home after alleged reports of a loud argument.

She moved into Downing Street when he took office in July, but kept out of the campaign for the December general election, in which Johnson secured the Conservatives' best result since the 1980s.

In February the couple announced they were engaged and were expecting a baby "in the early summer".