Time Warner has warned cable customers not to use their accounts to provide free or cheap internet access to others via wireless connectivity.
Illegal communities based around Wi-Fi are springing up in the US as users opt for high-speed access without being tethered to a telephone line or cable.
One person buys a cable access account and then lets others based nearby connect using wireless accounts.
Time Warner said it has identified a dozen users operating in this way. But a spokesman admitted that identifying them was difficult, as wireless connectivity is hard to trace.
AT&T Broadband said it is monitoring websites to find adverts for cheap or free wireless services and said that it would warn those who misused its network.
But it is possible that the neighbours may be tapping their link without their knowledge.
A wireless network within a home or apartments is available to all those within about 300 feet. There is security software that would require a password, but many US users do not use it.
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