An officer said the threat was: “We are telling Air China: immediately suspend operations of all flights. Otherwise we will bomb the planes. We will crash the airplanes into the site of the Olympics.”

“Two airplanes of Air China are currently under inspection at the Narita airport,” said the officer. “They are flights bound for Beijing and Shanghai.”

An official at Japan’s transport ministry confirmed the bomb threat appeared to be linked to the Olympics.

“Air China received an email saying a bomb will be exploded unless all Air China flights are suspended,” said the official, Toshiro Tachimoto, adding that the email contained a reference to the Olympic Games.

Air China officials in Tokyo and Beijing were not immediately available to comment.

One Air China flight bound for Chongqing via Shanghai was forced to return to Japan’s Nagoya airport due to the threat, an airport official said.

“Flight 406 had to return after the bomb threat. No one was injured. Once the safety of the airplane is confirmed, the plane will take off soon,” said Akiko Noguchi, a spokeswoman for Central Japan International Airport.

It was unclear how many people were aboard the plane at the time.

The threat came as China enforced sweeping security measures across Beijing to protect the Olympic opening ceremony, deploying thousands of armed police and locking down areas of the city centre.

Beijing airport was set to shut down just ahead of 1200 GMT start of the ceremony at the Bird’s Nest stadium to be attended by world leaders such as Chinese President Hu Jintao and US counterpart George W Bush.