Johnson, who has been married twice previously, this month finalised his divorce with estranged wife Marina Wheeler, with whom he has two daughters and two sons. Tonight, Downing St would not confirmed if the decree absolute had been granted.
While he was still married, he began dating PR guru and Tory adviser Symonds, and the couple are believed to have been in a relationship for over a year.
A spokesperson for the couple said tonight: 'The Prime Minister and Ms Symonds are very pleased to announce their engagement and that they are expecting a baby in the early summer.'
Symonds and Johnson, who live together in Downing St, recently enjoyed a luxury Caribbean vacation in Mustique.
The couple's relationship has been firmly in the public spotlight, and a row in her Camberwell flat during last summer's Tory leadership election wobbled Johnson's bid for the top job.
Yesterday, it emerged Symonds had hired a media professional to handle her communications and help on her campaigns as she moulds into the role of the UK's first lady.
But she has already been described as the power behind the throne, with reports of her and Downing St supremo Dominic Cummings in a tug of war for the PM's ear.
Symonds is the first unmarried partner of an incumbent British prime minister. She has largely kept out of the public eye since Johnson took the reins of the UK government in July.
She was scarcely seen at the PM's side on the general election campaign trail, but waved with her soon-to-be husband on the steps outside the famous black door following the Conservatives' thumping victory.
The couple chose to live at the larger four-bedroom flat at No 11 Downing St instead of the smaller two-bedroom official residence at No 10.
They already share a dog, Dilyn, with the PM saying that he helped keep his stress down during the election campaign.
It has been suggested in the past that Symonds could not become a fully-fledged "first lady" until the couple were married.
Johnson was coy when asked about the subject last year, telling reporters that marriage speculation was "a tiny bit premature".
Charlotte, James and even Winston are the bookies' favourites for potential baby names - with odds for the name "Brexit" at 1000/1.
Ex-Chancellor Sajid Javid, who was defenestrated by the PM in the recent reshuffle, was one of the first to offer congratulations to Johnson Symonds, who is his former special adviser.
Former Scottish Conservatives leader Ruth Davidson wrote: "Congratulations to Boris Johnson and Carrie Symonds."
Davidson, who had been critical of Johnson during the European referendum campaign, stepped down as Scottish leader last year to spend more time with her young family.
Tom Tugendhat, chair of the Commons foreign affairs committee, called the baby and wedding announcement "lovely news".
The Conservative MP said: "Congratulations to both Boris Johnson and Carrie Symonds. Happy news!"
But Johnson's political rivals questioned the convenience of the timing of the announcement, which comes as the government is taking fire for its slow response to the coronavirus outbreak, the PM going AWOL during the flooding crisis and bitter rifts at the top of the Home Office.
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.
And Labour MP Florence Eshalomi said it was 'very convenient' for the Prime Minister and Ms Symonds to make their announcement tonight.
Several political commentators tonight also mooted the announcement as a distraction ploy to the government's current woes.
Previous prime ministers David Cameron and Tony Blair both had Downing St babies, Florence in 2010, and Leo in 2000, respectively.
The last babies born to prime ministers before Leo and Florence arrived more than 150 years ago, when Lord John Russell's wife gave birth to two sons, George and Francis, during her husband's first stint in office between 1846 and 1852.
Johnson is first prime minister to be divorced in office in modern times, and his union with Symonds will be his third marriage.
Earlier this month, a court heard that the Prime Minister and his estranged second wife Marina Wheeler were preparing to end their marriage after reaching an agreement over money.
A family court judge approved a financial settlement and gave Wheeler permission to apply for a divorce decree.
Wheeler and Johnson, who have four children, separated in 2018 after marrying in 1993.
The PM was a childhood friend of Wheeler - the daughter of BBC journalist Charles Wheeler - when both were pupils at the European School in Brussels.
Johnson met his first wife, Allegra Mostyn-Owen, while they were students at Oxford. They married in 1987, but the marriage was annulled in 1993.
In 2013 it emerged during another court hearing that Johnson had fathered a daughter during an adulterous liaison while Mayor of London in 2009.
In 2004, he was sacked from the Tory frontbench over a reported affair with journalist Petronella Wyatt.
Boris and Carrie's "Brexit baby" - How Boris Johnson and Carrie Symonds made history as first unmarried couple to live together in Downing St.
Boris Johnson and Carrie Symonds made history as the first unmarried couple to officially live together in Downing Street.
The 55-year-old Prime Minister and 31-year-old conservationist, whose relationship has been the subject of intense intrigue, are now getting ready to welcome what may be referred to as a "Brexit baby".
Symonds, a former Conservative Party communications chief, found herself making headlines when she was romantically linked to Johnson last year just months after the announcement of his separation from second wife Marina Wheeler.
The public relations expert had been in the so-called Westminster Bubble for most of her remarkably ascendant career, a high-point being when she was made head of party PR aged just 29.
Symonds joined the Tory party media machine in 2009, first as a press adviser, then head of broadcast at Conservative campaign headquarters ahead of the 2015 general election.
Her association with Johnson dates back to the early years, having worked on his successful re-election bid at City Hall in 2012.
But a row that saw police called to their home in the early stages of the Conservative leadership race offered a glimpse into the complicated private life about which Johnson tries desperately to avoid answering questions.
The couple had been living together at Symonds's flat in Camberwell, south London, until the well-publicised row recorded by neighbours in June 2019.
They now live in a flat above Number 11 Downing St and have adopted a Jack Russell-cross puppy called Dilyn.
In December, Johnson's sister Rachel Johnson said she has yet to meet Symonds.
But their father Stanley, who appeared alongside Johnson on ITV's Good Morning Britain, said he has met Symonds, adding: "The Johnsons are united in this."
Back in January last year, Symonds was joined by Stanley Johnson at an event in London opposing Japan's plans to resume commercial whaling.
The pair were spotted smiling and chatting together after they both addressed the crowd.
Meanwhile, Johnson's turbulent marriage to Wheeler, during which they had four children, ended after 25 years together in September 2018.
In 2004 he was sacked from the Tory front-bench over a reported affair with journalist Petronella Wyatt and the Appeal Court ruled in 2013 that the public had a right to know he had fathered a daughter during an adulterous liaison while mayor of London in 2009.
Claims that Johnson squeezed the thigh of journalist Charlotte Edwardes, at a private lunch at The Spectator magazine's HQ shortly after he became editor in 1999, overshadowed his first Conservative Party conference as PM.
But in a public display of affection, Johnson kissed Symonds after his speech to party members at the event.
Johnson greeted Cabinet ministers before embracing his girlfriend, kissing her on the cheek and then walking out of the hall holding her hand.
Meanwhile, allegations about Johnson's relationship with American entrepreneur Jennifer Arcuri, and whether she enjoyed preferential treatment while he was London mayor, also dominated the headlines in September.
