Communications regulator Ofcom has revealed that both BT and TalkTalk are failing to comply with its Broadband Code of Practice by not voluntarily providing speed estimates to consumers often enough.
Ofcom established the Code of Practice in December 2008, which aims to provide broadband customers with transparency over the speeds they are likely to receive before signing up to any particular service.
Additional requirements were introduced to the Code in July 2011, which committed ISP signatories to give maximum speed estimates in the form of a range as early as possible in the sales process.
However, during a mystery shopper exercise, Ofcom found that some ISPs were under-performing in this area, where TalkTalk was 47 percent less likely than other ISPs to provide a speed estimate without being prompted by a caller, and BT Total Broadband 48 percent less likely.
Ofcom has discussed the issue with BT and TalkTalk, who have both agreed to address the problem by amending their staff training and sales processes.
ISPs that were most likely to give callers an estimate of speed without being prompted, although still nowhere near the 100 percent mark, were Karoo with 76 percent of cases, Sky with 72 percent and Plusnet with 67 percent.
Overall, speed estimates were provided in 59 percent of all calls without the caller having to prompt the sales respondent.
Ofcom will conduct further mystery shopping research next year to check whether improvements by the ISPs have actually been made and is also planning to update the Code of Practice over the next year.
"It is vital that as the choice of broadband services expands, UK consumers get the best possible information when choosing a broadband provider," said Claudio Pollack, Ofcom's consumer group director.
"We are working with internet providers to improve information that consumers receive when they sign up to a new service and will continue to monitor this area closely."