BBC News' Caroline Quinn: Blairs said to be surprised but delighted

Mrs Blair, 45 and her husband are said to be delighted at the news, though it came as a "total shock".

The child was almost certainly conceived during the family's month-long holiday in France and Italy earlier this year.

Mrs Blair discovered she was pregnant five weeks ago and told her husband, who is 46, on the eve of the Labour Party conference in Bournemouth. The baby is due in May.

Happy holidaymakers - the Blairs in August

The couple already have two sons, Euan, 15, and Nicky, 14 and a daughter, Kathryn, 11.

Politicians of all parties have welcomed the news.

Opposition leader William Hague and his wife Ffion sent their congratulations: "It is wonderful news. We are delighted for the whole family."

Liberal Democrat leader, Charles Kennedy, said: "Great news. My warmest personal congratulations and good wishes to both Tony and Cherie."

Few knew

Mr Blair's constituency agent, John Burton, 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."

Cabinet Office Minister Mo Mowlam also welcomed the news.

Speaking on BBC One's Question Time, she said: "Can I just say to Tony and Cherie, if you are watching, I am terribly happy for you both and for the rest of the family. I didn't know."

It is thought Mr Blair had been hoping to keep the impending arrival secret until after Christmas, but the news leaked out.

'Original plan was five'

The Prime Minister has always said his family are the most important thing in his life.

In July, shortly before the fateful holiday, he told BBC's Newsround he would have liked to have more children.

He said: "We would have had five if, well, if we had carried on, I suppose. I think big families are great."

According to Commons sources, the last Prime Minister to father a child while in office was Lord John Russell, whose fourth son George Gilbert William Russell was born in 1848.