The proud dad admitted he was “a little shocked” at the surprise arrival, with the baby originally due in December.
But yesterday he revealed on Twitter that Callum David Robert Hoy was born just after 2pm, and that both he and mum Sarra were doing well.
The 11-time world champion tweeted a picture of Callum’s tiny hand grasping his finger and praised “incredible” hospital staff.
He said: “Amazed, delighted and a little shocked to announce the birth of our beautiful baby son, Callum David Robert Hoy! Born at 29 weeks.
“Although very early days for wee Callum, he’s doing well as is Sarra. Huge thanks to the incredible medical staff.”
Sir Chris’ sister Carrie Hoy later tweeted: “So happy and excited to be an auntie!”
Fans rushed to congratulate the couple, who announced they were expecting their first child in June during a TV interview. Sir Chris, who retired last year, said in the interview the pregnancy had “happened at the right time”.
“Everybody I speak to when I ask about them and their experiences about becoming a parent, they mention lack of sleep, which I’m not great with, so that’ll be a big challenge,” he said. “I think maybe I’m just blindly optimistic that it’s going to be great fun and dead easy. I’m sure it won’t be, I’m sure there’ll be lots of tough things but I’m just very excited. It all seems to have happened at the right time.”
Famous faces from the worlds of entertainment and sport took to social media to wish the couple well.
TV presenter Lorraine Kelly tweeted: “Congrats! He is so teeny tiny. Hope the three of you are ok. Baby Callum knew how fabulous his mum and dad are, wanted to say hello as fast as possible! Lots of love to all of you.”
And the official Twitter account of the British Olympic team even seemed to have the day-old baby already pegged for future sporting success, tweeting: “A warm welcome to the world to, Callum #FutureOlympian.”
At ten weeks early, Callum joins only 8.5 per cent of babies who are born premature – with many arriving more than eight weeks early needing specialised care in a Neonatal Intensive Care Unit to help them breathe and feed.