Just under half (48 percent) of all web users have no idea what the download limit is on their broadband package, says uSwitch.com.
Research by the comparison website also revealed that 38 percent believe their internet access has an 'unlimited' download limit and is not subject to a cap.
As a result web users are placing themselves at risk of having their service limited, suspended or even terminated by their provider
uSwitch.com said on average Brits spend two hours a week using the internet to watch films, TV or play games.
Nearly one in ten (nine percent) admitted they had been contacted by their ISP regarding 'excessive' use, while two percent said they had been penalised.
The comparison website said, with the exception of Sky, the majority of UK ISPs imposed some kind of fair usage policy on 'unlimited' broadband packages.
"It seems ludicrous that Unlimited Broadband does not mean what is says on the tin. The term 'unlimited' should be banned on broadband adverts unless, like Sky, the service really has no limits," said Matt Wheeler, communications expert at uSwitch.com.
"In the meantime, millions of broadbanders are surfing the web and enjoying the delights of on-demand TV in blissful ignorance that they could well be approaching the limit that is acceptable to their provider."
uSwitch.com urged web users to check their fair usage policy and if in doubt, ask their provider to clarify the acceptable usage.