Mathew Davis, 22, was on the Boeing 777 flight from Tokyo to London with 150 passengers on board when he scrawled the message on a toilet door that said there was a live bomb on the plane.
It read: "The bomb on board will explode at 16.00GMT unless our demands are met" and was found by a terrified stewardess when the plane was 35,000 feet over the Middle East.
A previous hearing was told a cabin crew member was "so distressed when she read the message she was left shaking and terrified. She believed there was a bomb on board and she was going to die.
"She was constantly looking at her watch and thought her time was up."
The captain alerted the authorities and Davis was arrested when the plane safely touched down at Heathrow Airport at the end of its 7,000-mile journey. Police searched the jet, but no explosives were found on board.
Davis admitted a charge of communicating false information with intent when he appeared at Isleworth Crown Court.
Wearing a smart grey suit, white shirt and black tie, he spoke only to confirm his name and enter his plea.
The charge states that he "communicated to Sarah Jane Spencer information that a bomb was liable to explode at 16:00 hours, knowing or believing that information to be false, with the intention of inducing in Sarah Spencer the false belief that it was true."
Paul Lazarus, in mitigation for Davis, said a custodial sentence was likely but that his case had "exceptional mitigating circumstances" due to his young age and the death of his mother in childhood. He asked for a pre sentence report to be produced and for further psychological assessment.
Judge Anna Guggenheim QC said: "This young man has a history of highly unusual acts which could be described as attention seeking, but which are potentially dangerous.
"Quite evidently those around him when he caused them to fear that the plane was going to explode would have been extremely frightened, but whilst a prison sentence would provide punishment it does not address the risk of him doing something similar in the future."
She released Davis on conditional bail, with residence and cooperation in psychological assessments, telling him: "You have pleaded guilty at the first opportunity before this court.
"Your early plea of guilty and acceptance of responsibility for this serious offence will stand to your credit and your sentence will be less severe."
Davis, of Commonwealth Drive, Crawley, West Sussex, is due to be sentenced next month.
