Conservative leader David Cameron has said he is "really excited" after wife Samantha gave birth to their third child - a boy - by Caesarean section.
Mrs Cameron, 34, gave birth at St Mary's Hospital in Paddington, London, with her husband, 39, at her bedside.
Baby Cameron was delivered at 1155GMT, weighing 7lb 13oz. Both mother and child are said to be doing well.
The Camerons, who have been married since 1996, have a son Ivan, three, and a daughter Nancy, two.
No name yet
Mr Cameron told reporters outside the hospital that his new son had blue eyes and black curly hair.
He added that his wife was "sitting up" comfortably and said of the birth: "It was all over very quickly."
Mrs Cameron was said to be well
Mr Cameron said: "We are going to be here for a couple of days while we make sure everything is OK.
"We haven't got a name yet - I can't make decisions like that."
Asked whom the baby most resembled, he said: "All of us."
Mr Cameron had been in a meeting with his shadow cabinet in the House of Commons at 0845GMT when he got a call to say his wife was on her way to hospital.
The baby was born slightly earlier than the expected date by Caesarean for medical reasons - both previous births have been by Caesarean.
Health Secretary Patricia Hewitt told the Commons: "We all wish [Mr Cameron] well, both as a parent and in his efforts to give up smoking."
Ex-Tory leader William Hague, now shadow foreign secretary, will stand in for Mr Cameron at prime minister's questions on Wednesday.
Shadow home secretary David Davis will then be nominally in charge of the party when Mr Hague leaves for an engagement in Washington later that day.
Mr Cameron, Conservative leader since December, has said he plans to take up to two weeks' paternity leave.
He will take one week immediately and "probably" another week soon afterwards "when the baby isn't sleeping and Samantha's more tired", he said recently.
Commons leader Geoff Hoon said: "I am delighted that Mr Cameron, who I should congratulate on the birth of a child, has been converted to the cause of paternity leave.
"We all know that he voted against the provision when it came before the House."
He added: "But we always welcome a conversion and we seem to be welcoming a conversion of large parts of the Conservative Party to Labour polices. So long may it continue."
Former Liberal Democrat leader Charles Kennedy's wife Sarah gave birth to a son, Donald, during last year's general election campaign.
And Tony Blair's wife Cherie gave birth to a fourth child - Leo - in 2000. It was the first baby born to a serving prime minister for 150 years.
Mrs Cameron is director of Bond Street stationery firm Smythson.
The family divides its time between homes in west London and Chipping Norton in Oxfordshire.