Stan Lee attends the Los Angeles premiere of 'Thor' held at the El Capitan Theatre in Hollywood, California - May 2, 2011. (Nikki Nelson/WENN.COM)

Comic book legend Stan Lee has sparked fears for his health after pulling out of all his upcoming personal appearances due to a "very serious circumstance".

The 89 year old was scheduled to be a star guest at the Authors! Authors! series at Toledo-Lucas County Public Library in Toledo, Ohio on Thursday, but his visit has been cancelled.

His appearance at the Ohio Comic Con next weekend has also been axed.

Convention promoters have revealed they were "informed by Stan Lee's management that Stan would be unable to attend any upcoming appearances for the near future".

Toledo-Lucas Library spokesperson Susan Gibney told the Toledo Blade publication Lee's agent did not give a specific reason for the cancellation, other than to say it was for "a very serious circumstance".

Fans have since taken to social networking sites Facebook and Twitter to send their best wishes to Lee.