US TV
Rex Features
Show insiders reportedly told the programme that Clooney was scheduled to film his scenes later this week.
The 47-year-old actor rose to fame on the NBC medical drama playing Dr Doug Ross from 1994 until 1999.
Last week, the show's executive producer John Wells appeared to dismiss claims that the star would make a special appearance, saying: "I would be surprised...But then, George loves practical jokes. Who knows?"
The final episode will air on April 2 and will be preceded by a one-hour retrospective.
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