The Giant Mars e-mail chain letter says the red planet will pass close to Earth on 27 August.

Security software firm Sophos said people should delete the message and urged them not to believe everything they received via e-mail.

A spokesman said:

"Hoaxes and chain letters like Giant Mars are not harmless - they waste time and bandwidth, and can make support departments see red.

"Users need to be more sceptical, and ask themselves whether everything they are told by e-mail can be believed."