Labour leader Ed Miliband's partner Justine Thornton has given birth to their second child, a son.
The boy, weighing 8lbs 3oz, was born at University College Hospital in London.
Speaking outside the hospital Mr Miliband said his partner had done "absolutely brilliantly" and they were both "really delighted".
He added that he had a preferred name but it was "still being signed off". He said he was looking forward to taking two weeks' paternity leave.
Harriet Harman will stand in for him in Parliament.
A smiling Mr Miliband spoke to reporters outside the hospital where he thanked staff and paid tribute to Ms Thornton.
"We didn't know he would come quite as quickly as he did. Justine did absolutely brilliantly," he said, adding: "I have the easy part."
He said he was looking forward to introducing his 17-month-old son Daniel to his new brother: "He's really gorgeous, he looks a bit like me but as I say, we are absolutely delighted and overjoyed."
Among those sending congratulations to Mr Miliband - leader of Britain's official opposition - were his political rivals David Cameron and Nick Clegg - both of whom also have young children.
A Downing Street spokesman said the Prime Minister had sent his "very best wishes" to Mr Miliband and his partner. He sent a card welcoming the "wonderful news" and sending "heartfelt congratulations" from himself and his wife, Samantha.
Deputy PM Nick Clegg also sent his congratulations, adding he and his wife Miriam were "thrilled for them".
"It's been a hectic few months for them, so I hope they are given the time and space to fully enjoy their new arrival."
Mr Miliband and his partner Justine Thornton are not married although Mr Miliband said they planned to get married when questioned about it after becoming Labour leader in September.
He also admitted to being embarrassed that he was not named on the birth certificate of their first son but said he would be sure not to miss out a second time.