Boris Johnson and Carrie Symonds' son is the latest in a line of Downing Street babies

Boris Johnson and Carrie Symonds have welcomed a Downing Street baby. (Getty Images)

Congratulations are in order for the Prime Minister Boris Johnson and his fiancee Carrie Symonds who have announced the birth of their baby, a son.

The baby boy, who was born this morning, is the latest in a whole line of Downing Street babies to be born to a serving PM.

Downing Street has welcomed a number of babies in recent years, with Tony Blair and David Cameron both expanding their families while living at Number 10.

But that is a relatively recent trend, with a gap of more than a century before Leo Blair became a Downing Street baby in 2000.

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Tony and Cherie Blair welcomed baby Leo in May 2000 which Tony Blair was the serving PM. (Getty Images)
Leo was the first baby to be born to a serving Prime Minister for more than 150 years. (Getty Images)

Cherie Blair gave birth to little Leo three years after Labour’s 1997 general election win.

Mr Blair’s youngest son was the first child legitimately born to a serving prime minister for more than 150 years.

In Cherie’s memoirs, Speaking For Myself, the QC revealed Leo was conceived during a visit to Balmoral, the Queen’s residence in Scotland.

Leo was born at Chelsea and Westminster Hospital in London on May 20, 2000 at 00.25am, weighing 6lbs 12oz.

He joined elder siblings sister, Kathryn and brothers Euan and Nicky to become the youngest Blair baby.

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Before Master Blair’s birth, Lord John Russell and wife Lady Russell saw the last child to be born to the office holder of First Lord of the Treasury – the prime minister’s official title.

Lady Russell gave birth to two sons, George and Francis, during her husband’s first stint in office between 1846 and 1852.

After a century-and-a-half delay between prime ministerial babies, Downing Street only had to wait 10 years for the patter of tiny feet when then PM David Cameron and his wife Samantha welcomed daughter, Florence Rose Endellion Cameron, into the world in 2010.

David and Samantha Cameron on the steps of Downing Street with new baby Florence in 2010. (Getty Images)

Mr Cameron had only been in office just three months prior to his daughter’s birth, by Caesarean section at the Royal Cornwall Hospital in Truro while the Camerons were on their summer holiday.

She was Mr and Mrs Cameron’s fourth child, although their eldest son Ivan, who suffered from cerebral palsy and severe epilepsy, tragically passed away the year before sister Florence’s arrival.

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On learning the news that PM Boris Johnson had also welcomed a baby while serving in office, the former PM David Cameron headed online to offer his congratulations while jokingly apologising for not leaving the cot.

“but the climbing frame should still be in the garden!” he added.

While promoting his memoirs, For The Record, last year, Mr Cameron said Florence, who was only five when the family left Downing Street in 2016, was hazy about his time in power.

He told the Cheltenham Literature Festival that his daughter had asked him: “Daddy, is it true, were you actually the prime minister?”

Additional reporting PA.