Ms Symonds wrote on Instagram that they got engaged at the end of last year, adding she felt "incredibly blessed".

Mr Johnson, 55, and Ms Symonds, 31, became the first unmarried couple to occupy Downing Street when they moved in last year.

Ms Symonds is also the youngest partner of a prime minister in 174 years.

In a post on her private Instagram account, she wrote: "Many of you already know but for my friends that still don't, we got engaged at the end of last year... and we've got a baby hatching early summer. Feel incredibly blessed."

It will be a third marriage for Mr Johnson - who divorced his first wife and is now estranged from second wife Marina Wheeler.

Earlier this month, a court heard that the prime minister and Ms Wheeler were preparing to end their marriage after reaching an agreement over money.

Ms Wheeler and Mr Johnson, who have four children, separated in 2018 after marrying in 1993.

Ms Symonds, the former Conservative Party communications chief, was first romantically linked to Mr Johnson by the media in early 2019.

But her association with Mr Johnson goes back to when she worked on his successful re-election bid as London mayor in 2012.

She is now a senior adviser for US environmental campaign group Oceana, after quitting her role with the Tories in 2018.

Former chancellor Sajid Javid, who resigned from government after a dispute with Mr Johnson, offered the couple his congratulations, tweeting it was "wonderful news".

Former Scottish Conservatives leader Ruth Davidson was one of the first to congratulate the couple on social media.

Both Tony Blair and David Cameron had children while in office as prime minister.

Mr Blair and his wife, Cherie, welcomed their fourth child Leo, in 2000, three years after the Labour leader's landslide victory.

A baby daughter, Florence, was born to Mr Cameron and his wife, Samantha in 2010.