Ticketing systems, the transport network and hotel bookings as well as security are among potential targets, the report said.

Olympic security officials are also planning for the possible diversion of aircraft to protect airspace around the venues from terrorist attacks, the officials said.

This is normal practice for any Olympic host city.

The greatest threat to security at the Games is international terrorism, the Government's latest "Safety and Security Strategy" report said.

A spokesman said: "There's no current evidence of a terrorist threat to 2012.

"But if you look at precedents for sporting events, and to some degree about Olympic events, it would not be beyond the point of imagination to imagine a terrorist threat to 2012 nearer the time."

Metropolitan Police Assistant Commissioner Chris Allison said it was likely there would be a terrorist threat at the Games but he pointed to Britain's "long history of delivering safe sporting events".

Despite the Government yesterday lowering the threat level from international terrorism from "severe" to "substantial", security planning for the Games will be based on an assumed threat level of severe - the second highest level.

Home Secretary Alan Johnson said that security planning was "progressing in good time and to budget".

A total of £600 million has been put aside for security, but officials said if the threat increased it could put upward pressure on costs.

A potential cyber attack would pose a unique challenge because technology is constantly changing and more funds are being directed at the problem of computer attacks.

A spokesman said: "The general challenge reflected in cyber is anticipating what threats will look like three years out, and threats change, the nature of terrorism changes and the nature of serious crime changes as well, and cyber specifically is a really good example of a moving threat.

"I think we are very alive to the cyber [issue] and we are very alive to the fact that at the moment it is difficult to predict what it will look like with specific reference to the Games in 2012."

Officials are also drawing up plans for protecting water and air space around Olympic venues from possible attack, including possibly diverting aircraft.

It is expected diversions would most likely affect smaller, private aircraft.

The spokesman said: "We do expect there will have to be some management of air space.

"We do not expect that any airports will have to be closed."