Mobiles do not work on the London Underground at all
London Transport and mobile firms are warning people about an e-mail spreading rapidly containing inaccurate safety information.
The message claims that passengers on the London Tube system can contact emergency services via a satellite signal from their mobiles underground.
Mobiles do not work in the London Underground, and satellite signals cannot reach there either.
A Vodafone spokesperson said it had had numerous enquiries about the "hoax".
"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," a spokesperson from London Transport said.
"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."
A Vodafone spokesperson told the BBC News website that, at a time of heightened tension and security fears on the Underground system following the recent bomb attacks, it was clear people wanted to pass on useful information.
But, he added, people unthinkingly pass on e-mails without analysing the information contained in it.
"By forwarding false information they can panic people and also create vast amounts of work for people who have to deal with it and enquiries about it," he said.
Although the 112 number is indeed the European-wide standard number that will connect people to emergency services wherever they are in Europe, it is not accessible from mobiles underground in London.
A spokesperson from mobile operator Orange reiterated that if you do not have a signal on your mobile you simply cannot make a call.
If you are not in the Underground system and do not have a signal from your network operator, emergency calls can be made as your phone will automatically detect the next available network.
Orange and Vodafone both added that the line in the e-mail which says: "ALL phone companies have signed up" to some sort of satellite link up service was also inaccurate.
Vodafone said that often "hoax" e-mails contain accurate information, such as the 112 number's existence, to attempt to add credibility to the mail.
'Sick sense of humour'
It is unknown however, whether the e-mail has been devised as a hoax, intended to misguide people, or whether it is a result of misunderstood mobile technology.
"The e-mail originates either from ignorance or malicious intent, but the bottom line is that recipients should ignore its content and should resist the urge to pass on this information to others," Daniel Hepner, from web monitoring firm SurfControl, told the BBC News website.
"Either someone is attempting to cause confusion amongst commuters, or they have a very sick sense of humour.
"It's possible that it could stem from an individual that is unaware of the misinformation, but in the current climate all such e-mails will be treated with suspicion," he said.
Shortly after the bomb attacks in London, e-mail hoaxers tried to disrupt a campaign, In Case of Emergency (ICE), which aimed to get people to store next-of-kin details in their mobile phones.
The hoaxers claimed that the campaign was in fact a mobile phone virus. A further e-mail hoax circulated warning Underground travellers of further imminent attacks.
"The 'In Case of Emergency' virus hoax has heightened people's suspicions over this kind of traffic, and it is important that we all exercise caution before disseminating these messages further," said Mr Hepner.