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The 40-year-old, who lives with his wife and daughter Theodora - known as Teddy - in LA, wrote, "Ayda and I are delighted to announce that Theodora Rose Williams is going to be a big sister. RW xxx."
Robbie Williams and wife Ayda Field are going to give their daughter Teddy a little brother or sister.
Within a matter of hours, his message had been 'favourited' more than 6,500 times by his excited fans.
Robbie and Ayda were married in the grounds of his Beverly Hills mansion in August 2010 — as featured exclusively in HELLO! — and welcomed Teddy two years later, in September 2012.
Little Teddy - pictured as a newborn - is the apple of her daddy's eye.
The musician has spoken previously about the influence his "sensational" daughter has had on him.
"She's difficult to talk about," he said in an emotional interview with BBC Radio 4 in October. "Everything I do is for her."
HELLO! was on hand when Robbie Williams and Ayda wed in Beverly Hills in August 2010.
"Every time I open my mouth now in the studio it's a song about her, it would bore people to death but I could do a trilogy of albums about her."
"There's a reason for me to do everything now," added Robbie. "I wasn't going to have a kid and I wasn't going to get married then I met someone who was amazing and fell in love with her completely and I changed all my thoughts and feelings about that."