Ofcom has called on the UK broadband industry to improve the information provided to customers over download speeds.
The communications regulator said that although improvements have been made in this area in recent times, further work is still needed.
Ofcom conducted a mystery shopping exercise to check compliance with its Voluntary Code of Practice on Broadband Speeds, which was originally introduced in December 2008.
The code helps ensure consumers are aware of the likely broadband speeds they can get on their line from a broadband provider before signing up to a service.
A strengthened code came into force in July 2011, through which broadband providers have committed to give maximum speed estimates in the form of a range as early as practicable in the sales process.
In 93 per cent of telephone enquiries made by mystery shoppers, an estimate of the broadband speed was given by the provider.
And in 72 per cent of cases, this estimate was given in the form of a range.
However, some sales agents had to be prompted by mystery shoppers to provide speed estimates, rather than freely volunteering this information.
Download speed estimates were provided without prompting in 59 per cent of all calls, with Karoo (76 per cent of cases), Sky (72 per cent) and Plusnet (67 per cent) the best performers.
TalkTalk (47 per cent of cases) and BT (48 per cent) were less likely to provide a speed estimate without the caller requesting the information.
Both BT and TalkTalk have agreed to address this issue by amending their staff training and sales processes, Ofcom revealed.
Claudio Pollack, the regulator's Consumer Group Director, said: “It is vital that as the choice of broadband services expands, UK consumers get the best possible information when choosing a broadband provider.
"Our mystery shopping shows that, while consumer information about broadband speeds has improved in important areas, there is still more to be done."
He said Ofcom is working with broadband providers to improve information that consumers receive when they sign up to a new service and will continue to monitor this area closely.