01/22/2009 at 12:00 AM EST
has agreed to come back to the show that launched his career: ER.
Sources close to the series tell PEOPLE that ER
co-creator John Wells
has ordered a closed set to keep Clooney's appearance under wraps.
It's not known how many episodes will feature Clooney as the show films its final season. Neither NBC nor Warner Bros. reps would comment, nor did Clooney's publicist.
Clooney, himself, denied reports
last March he was returning.
Clooney's Dr. Doug Ross was last seen on ER
in a 2000 cameo walk-through in a remote lake scene with Ross's ex-Julianna Margulies.
Clooney is the most famous ER
alum and Wells has always wanted him to return for the show's last season. The 15th season was extended by an extra four episodes by NBC executives last week. -- Pamela Warrick