Boris Johnson and Carrie Symonds have announced they are expecting a baby and that they have got engaged.

The couple, who have been living together in Downing Street since Johnson became prime minister in July 2019, with Symonds becoming the first unmarried prime ministerial partner in history, are expecting the birth of the child in the early summer.

Details of the prime minister’s relationship with Symonds, 31, an environmental campaigner and former Conservative party official, emerged last year. The expected birth date of the child suggests the baby was conceived in autumn, around the time that the 31 October Brexit deadline was extended and it follows speculation over whether the couple would soon have a child together.

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.”

The couple’s relationship was the subject of scrutiny during the 2019 general election after police were called to Symonds’ home by a neighbour who claimed to have heard a row between the two. However, officers attended and confirmed both were “safe and well”.

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Johnson, 55, will become the first prime minister to get married in office for 250 years, while the last child born to an incumbent prime minister was Florence Rose Endellion, the daughter of David and Samantha Cameron, who was born in August 2010.

Johnson has repeatedly come under scrutiny over his personal life. In 2004, he was sacked from the Tory frontbench over a reported affair with the journalist Petronella Wyatt.

Appeal court judges ruled in 2013 that the public had a right to know that he had fathered a daughter during an adulterous liaison while mayor of London in 2009.

The prime minister is believed already to be the father of at least five children with two women – his second wife Marina Wheeler, whom he married in 1993, and art consultant Helen Macintyre, with whom he had an extramarital affair. He has always refused to say how many children he has in total.

Johnson refused to speak about his own children in November, nor the possibility that another might be on the way, after it emerged he had referred to single mothers as “irresponsible”.

Facebook Twitter Pinterest Carrie Symonds and Boris Johnson announce their engagement and baby on Instagram. Photograph: Apples_Symonds/Instagram

Challenged on live radio, he told LBC listeners: “I don’t think this is what the nation wants to hear. They want to hear how we’re going to get Brexit done.” Asked how many children he has, he replied: “I love my children very much but they are not standing at this election. I’m therefore not going to comment on them.”

Johnson announced that he was separating from his wife in September 2018 after months of being romantically linked with Symonds. Pictures of the pair on holiday emerged in March 2019 and the pair bought a house together in Camberwell in July 2019.

However, their association together dates back to when she worked on his successful re-election bid at City Hall in 2012. Symonds moved into Downing Street with Johnson when he became prime minister but has kept a low profile since then.

A family court judge on Tuesday approved an undisclosed financial settlement and gave Wheeler permission to apply for a divorce decree which will bring the marriage to an end.

Neither Johnson nor Wheeler, a barrister, were at the 10-minute private hearing in the central family court in London but both were represented by lawyers.

Johnson is believed to be the first prime minister to be divorced in office since the Duke of Grafton was granted a divorce, by act of parliament, while serving as prime minister in 1769.

Former Scottish Conservatives leader Ruth Davidson was among the first to congratulate the couple, while former chancellor Sajid Javid – who recently resigned after he was ordered to sack all of his advisers by No 10 – tweeted it was “wonderful news!”.

But Labour peer George Foulkes questioned the timing of the announcement from Downing Street on the same day as Home Office permanent secretary Philip Rutnam’s dramatic resignation.

The former MP of 26 years tweeted: “Wonder why they announced it today?”