The precise location of the historic event emerged after Alastair Campbell, the Prime Minister's official spokesman, dismissed newspaper reports that the newest Blair babe was conceived in Italy.
Mr Blair had been "completely, totally, 100 per cent stunned" by the news, Mr Campbell said as he revealed exactly how long the Prime Minister's wife had been in her happy state. He said: "She's 13 weeks pregnant, the baby is due in late May, which means the Tuscan connection is wrong."
Frantic consultation of diaries revealed that the Blairs had been on the third week of their summer break, at a chateau in St Martin D'Oydes in the remote south west of France, 13 weeks ago.
Mrs Blair appeared briefly before the massed ranks of the media camped out in Downing Street yesterday. She had broken the news to her husband that she thought she was pregnant in the week before the Labour Party conference in October. Mrs Blair was proved correct on the Monday of the conference, when she visited her GP in London.
Mr Campbell said: "He [Mr Blair] said, `I just can't believe that I'm going to be changing nappies, and this child is still going to be at school when I'm a pensioner'."
The couple were sorry that they had not been able to break the news to some of their friends and family before it leaked to the media. The Queen, however, was informed of the event "some weeks ago".
Mrs Blair had wanted to tell her father, the actor Tony Booth, in person after she was through the crucial first 12 weeks of the pregnancy. Mr Booth, 64, who was informed by reporters on Thursday night, said that every pregnancy was a worry, but he did not have any particular concerns. "She is my daughter, to me she is a kid. I don't think of her as a 45- year-old judge," he said.
Mr Campbell said that Mr Blair was likely to take paternity leave and would attend the birth in an NHS hospital. "At the moment, he [Mr Blair] normally gets woken up only because of Northern Ireland," Mr Campbell added.Reuse content