The Take That star was set to perform David Bowie's 1971 hit Life on Mars? during the August 12 finale, however he has cancelled his appearance as the date clashes with his pregnant wife's due date, reports The Mirror.
A source told the newspaper: "Robbie quit a few weeks ago. He had been lined up to perform solo material and it was hoped he would sing with Take That.
"However, just as rehearsals（排练） were kicking off, he said there was an unfortunate clash of dates."
Other artists rumoured to be performing at the closing ceremony include George Michael, The Who, Emeli Sande and Take That.
However, a spokeswoman for London 2012 has said: “We want our ceremonies to be a surprise. We've kept all acts that might be taking part under wraps.
"There's a lot of speculation flying about, but we're not netmenting on it."
Last week, Williams shared his father-to-be joy with fans by posting a picture of his pregnant wife's baby bump online.
The Angels hitmaker snapped Ayda at their LA home while she posed with her bulging belly.
He wrote "My two girls" on a message acnetpanying the photo.
Williams announced that his wife of almost two years was pregnant in March, later revealing that they were having a girl.