British Prime Minister Boris Johnson and fiancee Carrie Symonds announced the birth of a 'healthy baby boy', delivered at a government-run hospital in London on Wednesday.

The birth is believed to be slightly premature but both mother and baby are said to be doing very well.

"The Prime Minister and Symonds are thrilled to announce the birth of a healthy baby boy at a London hospital earlier this morning, a spokesperson for the couple said on Wednesday.

"Both mother and baby are doing very well. The PM and Symonds would like to thank the fantastic NHS (National Health Service) maternity team, the spokesperson said.

The news comes days after Johnson, 55, returned to 10 Downing Street on Monday after his hospitalisation for complications from COVID-19. He had been recuperating at his Chequers countryside retreat in Buckinghamshire, where he was joined by his pregnant fiancee.

Symonds had taken to Twitter earlier this month to express her relief at Johnson's discharge from hospital with a series of clapping hands emoticons alongside a picture of a rainbow, which has emerged as the symbol of support for the country's NHS through the course of the pandemic fightback.

The 32-year-old herself had been previously self-isolating with the couple's dog Dilyn after she developed symptoms of coronavirus but later revealed she was on the mend .

The couple had announced their engagement at the end of February, when it was also revealed that they were expecting their first baby in the summer.

The last child born to a sitting UK prime minister belonged to David and Samantha Cameron, who welcomed Florence Rose Endellion into their family in August 2010.

Symonds, who works for an ocean conservation charity, met Johnson during her time as the communications chief for the Conservative Party.