A new scam, in which fraudsters pose as legitimate ISPs to offer bogus tech support, either via the phone or on the net, is on the rise, the BBC has found.
It is a twist on an old trick which involved cold-calling a victim - often claiming to represent Microsoft - and charging for fake tech support.
The new variants have been spotted in the UK and US.
BT said that it is investigating the issue.
The online version of the scam involves a realistic pop-up which interrupts a victim's normal browsing session with a message that appears to be legitimate and seems to come from the victim's real ISP.
US security firm Malwarebytes has spotted several from US and Canadian ISPs, including ComCast and AT&T. It has also seen webpages created for UK ISPs, including TalkTalk and BT.