Walliams: I knew about Cowell baby
The 41-year-old Britain's Got Talent judge - who himself became a father in May - told The Sun how he had promised to keep Simon's secret.
David said: "I had this incredible information but it was a private thing, which I had to respect."
The comedian - who is a good friend of Simon's - said the music mogul will make a great father.
David said US socialite Lauren, 36 was "a lovely lady".
He added: "I'm thrilled for them. He'll be a fantastic dad, he's brilliant with kids.
"He's really generous with his time for them. He's seen as a bit of a Bond villain but he's quite cuddly."