A United Airlines flight returned to Sydney International Airport shortly after takeoff today following a bomb threat on-board.

Some 90 minutes into the flight, United Airlines flight 840, which had left Sydney for Los Angeles, returned to the Austrialian city after a note threatening to blow up the aircraft was discovered in the plane's toilet.

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At press time the Boeing 747 was on the tarmac, with all 246 passengers and crew on-board. The Sydney airport has been closed down under a security alert. Airport and other security personnel were to carry out a thorough search of the aircraft shortly, it was reported.

'The pilot made the decision to return to Sydney,' a spokesman for the Australian transport minister was quoted as saying. 'The pilot obviously felt the need to return to Sydney.'

A United Airlines spokesman was quoted as saying that 'an object found on board raised security suspicions'.