Boris Johnson's fiancee, Carrie Symonds, on Saturday said they had named their newborn son after their grandfathers and the hospital doctors who treated the British prime minister for COVID-19.

Wilfred Lawrie Nicholas Johnson was born on Wednesday, just days after his father returned to work following several weeks recuperating from the virus and a spell in intensive care.

Symonds, 32, posted a picture on her Instagram account of her cradling her son. 

"Introducing Wilfred Lawrie Nicholas Johnson born on 29.04.20 at 9am," she wrote.

"Wilfred after Boris' grandfather, Lawrie after my grandfather, Nicholas after Dr Nick Price and Dr Nick Hart -- the two doctors that saved Boris' life last month.

"Thank you so, so much to the incredible NHS maternity team at UCLH (University College London Hospital) that looked after us so well. I couldn't be happier. My heart is full."

The announcement of the birth came as a surprise as Symonds, a former Conservative party head of communications, was not thought to be due for several more months.

It is her first child while Johnson, 55, is believed to have at least five other children, including four with his second wife, Marina Wheeler, from whom he split in 2018.

Johnson was expected to take statutory paternity leave at some point in the future, his spokesman told reporters this week.

The couple are the first unmarried couple to officially live together in Downing Street and the baby is the third to be born to a serving British prime minister in recent history.

Tony Blair's wife, Cherie, gave birth to son Leo in May 2000, while David Cameron's wife, Samantha, had daughter Florence in 2010.