The couple have become proud parents to daughter Colette 'Coco' Josephine Williams via surrogacy after a ''long and difficult'' struggle to have another baby.
Taking to Instagram, she wrote: ''I spy with my little eye an extra little hand ... So we have been keeping a very special secret! We are delighted to share with you that we have had a baby girl....welcome to the world Colette (Coco) Josephine Williams!!
''It has been a very long and difficult path to get here, which is why we have kept it on the down low. Family comes in all forms, and this little lady, who is biologically ours, was carried by an incredible surrogate mother, to whom we will be eternally grateful. We are over the moon to have this beautiful baby girl in our lives and so blessed that we live in a world which makes this possible. As with Teddy and Charlie, we ask that you respect Coco's privacy and allow us to grow into our new team of 5! #TeamWilliams (sic)''
Meanwhile, Robbie - who has Theodora, five, and Charlton, three, with Ayda - previously revealed his daughter is a ''performer'' like her father.
He said: ''Ted is a performer. She came out of the womb with jazz hands! She should be doing stuff already because she is just naturally inclined to be a performer and I'm happy about that. It's a good job and a nice life and hopefully I can help her avoid the pitfalls I fell down ... I would love to [bring them on stage] all the time but we've chosen to keep the children out of public life. So it's difficult because the second I do that, it's an invitation to the world to put their faces in newspaper and right now we've got a good thing going because they can't do that.''