The news came after Dannii Minogue refused a USD 1 million deal, reported Daily Star online.
Williams, 38, is not the only singer who has been approached as Rod Stewart has also been signed up.
With auditions starting on Wednesday, the UK show's bosses have turned to some of the biggest names of music industry to fill the slot alongside Louis Walsh, Tulisa Contostavlos and Gary Barlow.
However, William's place on the panel is not intended to be a permanent thing, but instead he will be seen as a guest judge.
"There is always a buzz in the air when Robbie and Gary get together. This could save us. Obviously we love Gary, Tulisa and Louis but we wanted to kick off next week with a bang and get everyone talking," said an insider.
"Having Dannii back would have been huge. But that has fallen through so hopefully Robbie will join us and create the excitement we need to launch the series," he added.
