Video report by ITV News Correspondent Ivor Bennett
Boris Johnson and Carrie Symonds have announced that they are expecting a baby in the early summer and that they have got engaged.
A spokesperson for the couple said: "The Prime Minister and Miss Symonds are very pleased to announce their engagement and that they are expecting a baby in the early summer."
Mr Johnson, 55, and Miss Symonds made history as the first unmarried couple to officially live together in Downing Street when they moved in last year.
An early summer birth would suggest the new arrival was conceived during the Autumn, around the time that the October 31 Brexit deadline was extended.
The prime minister is thought to be a father to five children and the latest addition to his family will be his first child with 31-year-old Ms Symonds.
Mr Johnson was previously married to lawyer Marina Wheeler - the pair agreed a financial settlement earlier this month as part of their ongoing divorce proceedings.
The couple announced they had separated after 25 years together and were going through the process of divorce in September 2018 – saying that “as friends we will continue to support our four children in the years ahead”.
Carrie Symonds' association with Mr Johnson dates back to the early years of her career in the Conservative Party. She worked on his successful mayoral re-election bid at City Hall in 2012.
Ms Symonds and Mr Johnson, who live together at 10 Downing Street, recently arrived back from a luxury Caribbean vacation in Mustique. Their trip abroad caused controversy, with Labour calling for an investigation into who paid for the £15,000 holiday.
The couple's relationship has been firmly in the public spotlight, and a row in their flat during last summer's Tory leadership election grabbed headlines.
The couple's relationship previously made headlines when it had emerged police were called to the home they shared together in June 2019.
There was said to have been a loud altercation involving screaming, shouting and banging, according to the Guardian.
Scotland Yard said they were alerted to the situation by a caller who “was concerned for the welfare of a female neighbour”.
Former Scottish Conservatives leader Ruth Davidson was one of the first to congratulate the Prime Minister on the news that he and fiance Carrie Symonds are expecting a child.
Re-tweeting the news, the MSP wrote: "Congratulations to Boris Johnson and Carrie Symonds."
Ms Davidson, who had been critical of Mr Johnson during the European referendum campaign, stepped down as Scottish leader last year to spend more time with her young family.
Former chancellor Sajid Javid, who resigned from Government after a dispute with Mr Johnson, offered the couple his congratulations, tweeting it was "wonderful news!"
But a Labour peer has queried the timing of the announcement from Downing Street.
The Prime Minister's personal news follows the sensational resignation of Sir Philip Rutnam, the most senior civil servant at the Home Office, amid an alleged bullying row with Home Secretary Priti Patel.
Lord Foulkes of Cumnock, a former MP of 26 years, said on Twitter: "Wonder why they announced it today?"
Labour MP Charlotte Nichols tweeted: "Congratulations to the happy couple notwithstanding, it's truly remarkable that the announcement JUST HAPPENED to be on the day of the unprecedented resignation of top Home Office official Sir Philip Rutnam who will be taking a case of constructive dismissal against the Govt."
The couple's announcement came just hours after the top civil servant at the Home Office dramatically quit his post and launched a blistering attack on Home Secretary Priti Patel.