A German computer scientist says he's cracked and has published details of the code that keeps most mobile phone conversations private.
Karsten Nohl says he and other experts spent the past five months working out the code, used to encrypt codes that use GSM technology.
Mr Nohl said the work showed that GSM security was inadequate and he was trying to inform people about the widespread vulnerability. He says it can take a few minutes to decrypt a call.
"A few minutes after the call happened you can then listen in on it.
"It's clearly illegal to do so; it's not illegal to say that this is already happening."
The GSM Association, which devised the code, says the research is a long way from a practical attack on the technology. It says work is already under way to upgrade security.