Consumer watchdog Which? claims as many as three-quarters of households across the UK are not receiving the broadband speeds promised by advertisements.
A report from the consumer magazine found that as many as 15.4 million households had paid for packages with advertised speeds that were unachievable.
Tests found that only 17 per cent of homes were achieving advertised speeds, with that number sinking lower during peak evening hours.
Rural homes were found to be the worst affected, with 98 per cent of households unable to access the top connection speed advertised by their provider.
The findings come after industry regulator Ofcom unveiled measures making it easier for customers to switch their TV, phone and broadband providers.
Which? Executive Director Richard Lloyd said: "We want Ofcom to ensure consumers get the speeds promised by providers.
"It is not good enough that millions of homes are so poorly served by their broadband provider with speeds that just don't live up to what was advertised."
Current rules mean providers can only advertise a top speed if it is accessible to ten per cent of customers, but the watchdog's research found that some packages could not even reach that level of coverage.
It claimed that just four per cent of customers on TalkTalk's 17Mbps package were getting the top advertised speed while the rate for the 76Mbps deal was just one per cent
However, both companies have disputed the findings, with a TalkTalk spokesman stating: "Our data, based on over half a million customers, which far exceeds Which's base of a few hundred, shows that TalkTalk homes can achieve speeds beyond 17Mbps.
"We're compliant with the advertising guidelines and if they change, we will continue to comply. Our network is faster and more resilient than ever and we continue to work hard to further increase broadband speeds."
BT stated: "We're very clear that customers should not rely on headline claims, but instead use the personal speed quote we give them at the point of sale, which is based on their own line.
"If they aren't happy with this personalised speed they can decide not to buy from us; if they are happy with the speed, but find they don't achieve it, we allow them to end their contracts in line with the Ofcom code of practice."