TalkTalk has been investigated by the Advertising Standards Authority (ASA) after a customer complained about how the company advertised its broadband speeds.
The customer approached the watchdog after viewing a speed checking service on TalkTalk's site. After he entered his postcode, he was told: "Your estimated speed 3.8 Meg Your estimated speed range is between 2.1 and 5.3meg ...".
He challenged whether the ad was misleading, because he was a TalkTalk customer and had been informed that the maximum speed available to him was less than 2.1 Mbit/s.
TalkTalk claimed that it was compliant with the Ofcom Voluntary Code of Practice on broadband speeds, which stated that internet service providers must "provide a facility (line checker) on their website so that consumers can find out, in a clear and easily accessible manner, what their estimated access line speed is".
It added that given the material differences between the network measured access line speed and the throughput speeds consumers were likely to receive, the Code of Practice also required that ISPs explain that the actual throughput speed received would be influenced by a number of factors.
TalkTalk said it provided people with a link on its site that explained how it estimated speed. It also claimed that the speed checker results were based on the methodology set out in the Code of Practice, which was based on the standard distribution of access line speeds shown across their network but limited to between the 20th and 80th percentiles.
It said it had ensured it gave consumers a clear picture with its speed checker, which "was qualified, with the prominent statement that the speed was an estimated one as well as the text "Your estimated speed range is ...", for that reason."
However, it did admit that it could implement changes to improve the consumer experience in relation to the speed checker.
The ASA said TalkTalk didn't go far enough to explain these speeds and differences - and that the ad was misleading.
It ruled that the ad must not appear again in its current form and also told TalkTalk it must ensure its speed checker results were more clearly qualified in the future.
It also told the company to ensure it was in a position to provide evidence to substantiate the impression that was likely to be taken from its future advertising.