The news that Mrs Blair, 45, is expecting a baby - their fourth - was greeted by delighted congratulations from the Prime Minister's cabinet colleagues and his political opponents.
The baby, which is thought to have been conceived during the family's summer holiday near Pisa in Italy, will be the first born to a serving Prime Minister this century and is certain to raise questions about Mr Blair's long-term readiness to use Downing Street as the family home.
Confirming the news before it was leaked in a newspaper, a Downing Street spokesman said: "The Prime Minister and Mrs Blair had hoped to announce this later in the pregnancy as only a small number of family and friends and No 10 officials are aware of it. They are delighted at the news, which came as a total shock. The baby is expected in May." He added: "They are sorry they have not been able to tell all their friends before it has become public in this way."
The birth of a child is likely to highlight the Government's policies for children in the spring Budget, which the Chancellor has indicated will include more measures to support families.
Due in May - the third anniversary of Mr Blair winning power - the baby may complicate the Prime Minister's plans to go for early election in the following spring, when the baby is a year old. But it will reinforce his commitment to the family - raised in his maiden speech to the Commons - with impeccable timing in the run-up to the general election.
When told of the pregnancy, Mo Mowlam, the Cabinet Office Minister, said: "How wonderful." In Scotland, Donald Dewar, the First Minister, was "delighted to hear the news". And Gordon Brown, the Chancellor, said: "I am really pleased for the whole family. It is wonderful news."
John Burton, constituency agent for Mr Blair in Sedgefield, said: "The whole constituency will be thrilled. The first thing my wife said when she found out was that it explained why Cherie looked under the weather when we saw her last."
Charles Kennedy, leader of the Liberal Democrats, said: "Great news. My warmest personal congratulations and good wishes to both Tony and Cherie. I'm sure everyone will be delighted for them." And from the Tories, William and Ffion Hague said: "Congratulations to Cherie and Tony. We are delighted for the whole family."
Mr Blair, 46, has emphasised his wish to provide a normal family upbringing for his three children - Euan, 15, Nicholas, 14 and Kathryn, 11 - in spite of living "above the shop" at Downing Street. The last time a Prime Minister became a father while in office was in 1848 when the wife of Lord John Russell, 56, gave birth to a boy.
The Blairs may not wish to stop at four children. In July, the Prime Minister told Siobhan Maher, 13, who had won a competition to interview him for BBC1's Newsround: "We would have had five if, well, if we had carried on, I suppose." When Siobhan, of Solihull, West Midlands, said she was one of five, Mr Blair said: "I think big families are great."
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