TalkTalk has been sanctioned over a recent advertising campaign, which the regulator found to have misled consumers.
The Advertising Standards Authority (ASA) upheld several complaints from rival broadband provider BT and two members of the public against a press, TV and poster advert released in September 2011.
In the advert, TalkTalk claimed to provide the UK's "safest broadband", due to the fact all customers receive access to its HomeSafe service.
TalkTalk attempted to justify the claim on the basis that no other broadband provider offers in-built security features at the network level.
However, the ASA said members of the public could interpret "safest" as referring to a number of features, such as anti-virus protection or hacking prevention.
Indeed, the regulator said HomeSafe only offers a basic range of security features, and consumers would be unlikely to understand what network level security means.
The ASA warned TalkTalk that its security adverts must not appear again in their current form.