Boris Johnson follows modern trend of having a baby while in Downing Street

By Patrick Daly, PA Political Correspondent

Boris Johnson is not the first prime minister to welcome a baby into Number 10.

Tony Blair walked into Downing Street in 1997 as a father of three but had added to his family by the end of his decade-long premiership.

His wife Cherie Blair gave birth to son Leo three years after the then-Labour leader’s first landslide 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.

Tony Blair pictured with youngest son Leo and wife Cherie after his 2001 general election win (Toby Melville/PA)

Now 19 years old, Leo was born at Chelsea and Westminster Hospital in London on May 20 2000 at 00.25am, weighing 6lbs 12oz.

He joined sister Kathryn and brothers Euan and Nicky in the Blair sibling ranks.

Before Mr Blair, it was Lord John Russell and wife Lady Russell who produced 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 to return to its corridors.

David Cameron holding his daughter, Florence Rose Endellion Cameron, when she was a baby (Stefan Rousseau/PA)

Conservative prime minister David Cameron and his wife Samantha welcomed daughter Florence Rose Endellion Cameron into the world in 2010.

Mr Cameron had only formed a coalition government with the Liberal Democrats three months prior to her being born, with the general election that year producing a hung parliament.

Florence was delivered 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, died the year before sister Florence’s arrival.

While promoting his memoirs, For The Record, in October, 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 only a month previously: “Daddy, is it true, were you actually the prime minister?”

Mr Johnson will be hoping his and girlfriend Carrie Symonds’s baby is old enough to vividly recall their time in No 10 when his reign comes to an end.