British Prime Minister Boris Johnson and his girlfriend Carrie Symonds announced on Saturday that they are expecting a baby and have got engaged.

"The Prime Minister and Miss Symonds are very pleased to announce their engagement and that they are expecting a baby in the early summer," a spokesperson for the couple told Britain's domestic Press Association news agency. 

A Downing Street spokeswoman confirmed the news to AFP without providing further details.

Twice-married Johnson, 55 -- who split from his wife of 26 years in 2018 -- has been publicly dating 31-year-old Symonds since early last year. 

The British leader, who took office last summer and then swept to an emphatic victory in the country's December general election, will be the first UK prime minister to wed while in office for 250 years.

He and Symonds already made history as the first unmarried couple to occupy the prime minister's official residence.

Symonds, a former Conservative party spin doctor until now dubbed Britain's "first girlfriend", has kept a low profile since moving into Downing Street.

An avid environmental campaigner, she previously worked on his re-election campaign as London mayor in 2012 before becoming the ruling Conservative Party's director of communications.

Johnson already has four children through his second marriage to Marina Wheeler -- his eldest daughter is just five years younger than Symonds.

He also has a daughter resulting from a 2009 fling.

During a court battle relating to that birth, it was alleged he had had another child out of wedlock, but it remains unclear and Johnson has refused to comment.

Earlier this month it was announced that he and Wheeler, a lawyer, had reached a financial agreement over their imminent divorce. 

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