The Labour leader, Ed Miliband, and his partner Justine Thornton have announced the birth of their second child.
The boy arrived late last night, weighing 8lb 3oz, according to a party spokesman.
Ed Miliband passed on his thanks to well-wishers, tweeting: “Thanks for all the good wishes. He’s gorgeous. Mum and baby doing great.”
A Labour party spokesman said: “Ed and Justine are overjoyed at the birth of their second child and can’t wait to introduce the new arrival to his big brother, Daniel.
Mr Miliband spoke briefly to reporters as he left the University College Hospital today, saying: “We didn’t know he would come quite as quickly as he did. Justine did absolutely brilliantly.
“He’s really gorgeous, he looks a bit like me. As I say, we are absolutely delighted.”
Asked if the couple had decided on a name, Mr Miliband joked that he had his own views but “it’s still being signed off”.
Mr Miliband is expected to make sure he is named on this child’s birth certificate, after admitting he was “embarrassed” to have forgotten to register on 18-month-old Daniel’s records.
Mr Miliband will take now take two weeks of paternity leave from Parliament, with Labour’s deputy leader, Harriet Harman, covering his duties.
Mr Miliband has said he and Ms Thornton – who is a barrister – will “get around to” marrying “at some point”.
Deputy Prime Minister Nick Clegg passed on his congratulations on the “fantastic news”.
“Miriam and I are 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.”