The 29-year-old actress and husband Laurence Fox "couldn`t be happier" after welcoming their second child into the world on Thursday (05.04.12), and the actor has taken to twitter to reveal everything is "fine".
He tweeted: "One born every minute. We had our minute today. Couldn`t be happier. Beautiful boy. Everything fine."
The couple - who also have three-year-old son Winston - are yet to reveal a name for their latest addition to the family.
Following the birth, the former `Doctor Who` beauty`s sister Ellie also tweeted her congratulations to the pair.
She wrote: "Would like to say welcome new licckel baby boy I love you so much already can`t wait so kiss those big lips :) x (sic)"
In November, Billie offered to withdraw from her theatre role in Neil LaBute`s London play `Reasons To Be Pretty` after discovering she was expecting, but director Michael Attenborough was so keen for the blonde beauty to remain in the production that he re-wrote the script to include her pregnancy.
He explained: "Billie was Neil LaBute`s and my first choice for the part - and fortunately the character is pregnant - so with a bit of judicious re-writing we`re all delighted she`ll be with us.
"I told her that I`d love her to do it and she just wept on my shoulder. She thought that was it. She was thrilled."