A misleading spam e-mail has been widely circulated which claims that by dialling 112 you can get through to 999 even without having an available signal on your mobile phone - this is completely untrue.

Dialling 112 only works if you have a signal on your mobile phone

A London Underground spokesperson said: "This e-mail is incorrect.

"The 112 number does link people through to 999, but it only works if you have a signal on your mobile phone.

"If you have no signal bars on your phone, it will not work.

"It will not divert to a satellite signal.

"Even with a satellite mobile phone (which very few people have), you would need to have a clear line-of-sight to the satellite.

"You would have to be outside, not in a building or a Tube tunnel.

"The 112 number is the European equivalent of 999."