Samantha Cameron: "We're obviously delighted"
David Cameron's wife Samantha is expecting a baby in September, a Conservative spokesman has said.
The Tory leader, 43, and his 38-year-old wife are said to be "completely thrilled". They already have a daughter Nancy, six, and son Arthur, four.
Their older son Ivan, who had cerebral palsy and epilepsy, died at the age of six last year.
Mrs Cameron said earlier this month she was ready to help her husband in the general election campaign.
As she arrived home on Monday she told waiting reporters and photographers she was "very, very delighted as you can imagine".
The prime minister's official spokesman said Gordon Brown and his wife Sarah were "very pleased" to hear the Camerons' news.
"They will be sending a message of best wishes to them."
A spokesman for Nick Clegg said the Liberal Democrat leader had sent a message of congratulations and was "delighted" by the news.
Mrs Cameron has, until recently, kept a fairly low public profile, concentrating on her role as creative director of luxury stationers' Smythson.
But she gave her first television interview earlier this month, in which she talked about the family's home life and her relationship with the Tory leader.
Mrs Cameron described him as a "fantastic dad", but admitted he had some bad habits including making a "terrible mess" when cooking.
The couple's son Ivan died in February 2009 and Mr Cameron told his party's conference in October that the loss had made him ask "do I really want to do this" about his political career.
But he has admitted to wanting another child several times in recent months.
Answering a question on population growth at the Woodstock Literary Festival in September, he said: "I don't believe Britain is over-populated. 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."
If Mr Cameron were to become prime minister after the forthcoming general election, it would continue a modern trend for babies in Downing Street.
Both Mrs Brown and Tony Blair's wife Cherie had babies while living in Downing Street.
When he entered the world in May 2000, Leo Blair became the first baby to be born to a serving British prime minister for more than 150 years.
In October 2003, while her husband was chancellor, Mrs Brown gave birth to a baby boy, John. The couple had another child, James Fraser, in July 2006.
But like the Camerons, the Browns have also lost a child - their first baby, Jennifer Jane, died in January 2002 when she was just 10 days old.