Ofcom has urged broadband providers to give consumers clearer information about internet speeds.
Communications regulator Ofcom conducted a mystery shopping exercise and found TalkTalk and BT Total Broadband were significantly less likely than other providers to provide a speed estimate without prompting.
"It is vital that as the choice of broadband services expands, UK consumers get the best possible information when choosing a broadband provider," said Ofcom's consumer group director Claudio Pollack.
"Our mystery shopping shows that, while consumer information about broadband speeds has improved in important areas, there is still more to be done."
The compliance check with Ofcom's voluntary code of practice on broadband speeds found information given to consumers has improved.
The code helps to make consumers aware of the broadband speeds they are likely to get on their line from an internet service provider (ISP) before signing up to a service, with signatories committing to give maximum speed estimates in the form of a range as early as practicable in the sales process.
An estimate was given by the provider in 93 per cent of telephone inquiries, with 72 per cent giving the figure in the form of a range.
Overall, speed estimates were provided without prompting in 59 per cent of all calls despite the code requiring that the information should be volunteered as early as possible in the sales process.
Providers who were most likely to give callers an unprompted estimated speed were Karoo (76 per cent), Sky (72 per cent) and Plusnet (67 per cent).
TalkTalk and BT were significantly less likely to offer an estimate without prompting from the caller, at 47 per cent and 48 per cent respectively.
Both BT and TalkTalk have agreed to amend their staff training and sales processes, Ofcom said.
The code was last updated last year and Ofcom said it would review it again over the next year to ensure that it is still appropriate for consumers' needs.
"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," Pollack added.