Former UK prime minister Boris Johnson has become a father again, his wife Carrie said on Tuesday, with the couple's third child, and the ex-leader's eighth, born on July 5.

"Welcome to the world Frank Alfred Odysseus Johnson born 5th July at 9.15 am," Carrie Johnson wrote on her Instagram page, accompanied by a picture of her holding her new baby.

She joked "Can you guess which name my husband chose?!" in reference to Johnson's well-known love of ancient Greek myths.

"Am loving every minute of the sleepy baby bubble. Seeing my older two embrace their new brother with such joy and excitement has been the most wonderful thing. We are all very smitten," she added.

Their first son Wilfred was born in April 2020, not long after Johnson was treated in hospital intensive care for Covid.

Daughter Romy then arrived in December 2021, again when Johnson was still Britain's prime minister.

He was ousted the following summer, amid a revolt within his ruling Conservatives after a series of scandals, including Covid lockdown-breaking parties at Downing Street.

The 59-year-old Johnson quit as a Tory lawmaker last month after MPs found that he had lied to parliament over his knowledge of the parties.

Johnson, who has been married three times, has four children from his second marriage to the lawyer Marina Wheeler, and now has three with Carrie, 35, a former Conservative Party media adviser whom he wed two years ago.

The ex-leader has another child, a daughter, from an extra-marital affair.

In September 2021, before Romy's birth and after years of speculation about how many children he had, Johnson confirmed to US TV network NBC in an interview that at that time he had six.