The regulator has revealed further findings from the survey it conducted in March this year, which looked at what ISPs do to inform Brits of the broadband speeds they are likely to achieve when signing up for an internet connection.
During a mystery shopper test, BT provided the speed a line would be capable of without being prompted to just 28 percent of callers while Sky gave this information 39 percent of the time.
O2 came top in this category, providing the information without being prompted to 58 percent of the calls during the mystery shopper test.
However, when added to the number of callers who had to ask the ISP to provide the maximum speed a line is capable of O2 came out the worst, providing just 80 percent of mystery shoppers with the information.
Plusnet perfomed the best, with 93 percent of callers obtaining the information.
Ofcom also said that during the mystery shopper session, in total 15 percent of callers failed to obtain a likely speed for their line, while 42 percent had to prompt the ISP to provide the information.
Furthermore 40 percent were informed the speed was an estimate and 31 percent were warned that an actual speed depends on a number of factors.
"Consumers are now receiving more accurate information at the point of sale about their broadband service but our mystery shopping research reveals there is still significant further progress to be made, particularly in relation to the checkers used to calculate line speeds," Ed Richards, chief executive of Ofcom, said in March.
Ofcom plans to work with ISPs on tightening Voluntary Code of Practice on Broadband Speeds, in particular creating a universal speed measuring test, in a bid to ensure Brits are provided with more consistent and accurate information by Summer 2010.
The Code of Practice, which came into force in December 2008, has been signed by over 95 percent of UK ISPs.
Under the code, ISPs are required to provide consumers with information covering a number of areas before they sign a broadband contract.