David Cameron's wife, Samantha, is expecting a baby, due in September, the Conservative party announced today.
If Cameron becomes prime minister, he will be the third in a row to live in Downing Street with young children.
Tony Blair was the first prime minister in 150 years to father a child while in office when Leo Blair was born in 2000. Gordon Brown has two young sons.
The Camerons were said to be "completely thrilled" by Mrs Cameron's pregnancy.
The Tory leader has recently indicated in interviews – including a recent TV quiz by Alan Titchmarsh at a time when he must have known of his wife's pregnancy – that he would be happy to have another child.
A Conservative spokesman said: "The baby is due in September and David and Samantha are completely thrilled."
Answering a question on population growth from an audience member at the Woodstock Literary Festival in September, Cameron said: "I don't believe Britain is overpopulated. I don't have any plans to reduce it.
"I would quite like to add to it, personally, by quite possibly one, at some stage in the future."
Touring a maternity unit in West Yorkshire in December he met a nine-hour-old baby and told reporters: "It makes me want another one."
Mrs Cameron has been given a more high profile role recently, and the Conservative leader has described her as his "secret weapon" in the election campaign.
She made her television debut last Sunday as part of an ITV programme on her husband, Trevor McDonald meets David Cameron.
The Tory leader and his wife have two children, Nancy, six, and Arthur Elwen, four. Their son Ivan, who had cerebral palsy and epilepsy, died in 2009.
Gordon and Sarah Brown were "very pleased" to hear the news of Mrs Cameron's pregnancy, Downing Street said.
"They will be sending a message of best wishes to them," the prime minister's spokesman said.
