The British Prime Minister Boris Johnson's partner Carrie Symonds is pregnant and the couple have got engaged, they have announced.

"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 said.

On her Instagram account, Ms Symonds said: "I wouldn't normally post this kind of thing on here, but I wanted my friends to find out from me.

"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."

In a tweet, former UK chancellor Sajid Javid, who resigned after a dispute with Mr Johnson, congratulated the couple, saying it is "wonderful news".

Former Scottish Conservatives leader Ruth Davidson wrote on Twitter: "Congratulations to Boris Johnson and Carrie Symonds."

Congratulations to @BorisJohnson and @carriesymonds 👶 https://t.co/s7vbbphDcb

— Ruth Davidson (@RuthDavidsonPC) February 29, 2020

But not all politicians were so fulsome.

UK Labour MP Florence Eshalomi queried the timing of the announcement, appearing to reference Philip Rutnam's resignation from the British Home Office some hours earlier.

She tweeted: "Very convenient for this news to be announced today."

Very convenient for this news to be announced today 🤔🤔🙄🙄 🍼 🍼

— Florence Eshalomi MP (@FloEshalomi) February 29, 2020

The couple have been living together in 10 Downing Street since July and have adopted a dog, Dilyn.

Mr Johnson will become the first British prime minister to marry while in office for more than 250 years.

The 55-year-old already has four grown-up children with his second wife Marina Wheeler - the pair married 25 years ago but are now separated.

The couple have reached a financial settlement.

Mr Johnson also has a daughter with art consultant Helen Macintyre.

There has been speculation that Mr Johnson has fathered another extra-marital child.

Asked how many children he had during the general election in November, he told LBC: "I love my children very much but they are not standing at this election. I'm therefore not going to comment on them."

When pushed during his radio interview on whether he was expecting another, Mr Johnson refused to "get into discussions" on the matter.

He also insisted he meant "no disrespect" when he described single mothers as "irresponsible" in a magazine column published almost 25 years ago.

British Prime Minister Boris Johnson kisses his partner Carrie Symonds, after delivering a speech at Conservative Party conference in Manchester, England | Image: Stefan Rousseau/PA Wire/PA Images

Johnson and 31-year-old Symonds were thrust into the limelight during the summer Conservative Party leadership election.

Police were called to their house in London in the middle of the night.

A newspaper reported neighbours heard a loud altercation involving screaming, shouting and banging.

London's Metropolitan Police said the caller "was concerned for the welfare of a female neighbour" but officers attended and confirmed both were "safe and well".

Mr Johnson met his first wife, Allegra Mostyn-Owen, while they were students at Oxford University and they married in 1987.

The marriage was annulled in 1993 and he wed Ms Wheeler later that year.

In 2004, Mr Johnson was sacked by then Conservative Party leader Michael Howard following a reported affair with journalist Petronella Wyatt.