The labour Party leader Ed Miliband said yesterday that he was "absolutely delighted" after the birth of his second son. His partner, Justine Thornton, gave birth to a boy weighing 8lb 3oz (3.7kg) on Sunday night.
Mr Miliband's spokesman said he was at the hospital, near the couple's home in north London, when the baby arrived. The boy has yet to be given a name.
The 40-year-old Labour leader is expected to ensure 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 has said he and barrister Ms Thornton, also 40, will "get around to" marrying "at some point".
The Labour leader said the new addition to the family was "gorgeous".
Replying to well-wishers via social networking site Twitter, he wrote: "Thanks for all the good wishes. He's gorgeous. Mum and baby doing great."
Mr Miliband spoke briefly to reporters as he left the hospital today, saying: "We didn't know he would come quite as quickly as he did. Justine did absolutely brilliantly."
He said of his new son: "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".
He said: "We will be announcing the name in the coming days."
Deputy Prime Minister Nick Clegg said: "Congratulations to Ed and Justine on the fantastic news of the birth of their second son. 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."
A Downing Street spokesman said the Prime Minister had sent his "very best wishes" to Mr Miliband and his partner.
Mr Cameron wrote in a card: "Samantha and I would like to send you both our heartfelt congratulations on the birth of your baby boy.
"This is wonderful news and we could not be more pleased for you and your family."