BT wireless internet users are open to hackers, reports the Daily Telegraph.
Criminals can use easily downloadable software from the internet to crack passwords and change settings on the company's Home Hub wireless broadband system, reports the paper.
BT told the paper that the threat was "theoretical" and that hackers would have to "win the computer cracking equivalent of the National Lottery" to succeed.
But Paul Vlissidis, technical director of IT consultancy NCC Group, told the Telegraph that hackers could simply park outside a victim's house and search for their private network using a standard laptop to take advantage of the flaw.
BT said the risk was "being blown out of all proportion" and advised customers to check their website for advice.
NCC argued that most users were unwilling to change the settings on their networks.
It is understood that customers with version 1.5 or later of the BT Home Hub system are safe.