According to consumer watchdog Which?, UK households aren't getting the top broadband speeds as promised by advertisements.
The report suggested that up to 15.4 million households were paying for packages with advertised maximum speeds they would not achieve.
Ofcom has begun a broadband crackdown, making it easier for people to swap providers if they are unhappy with the speed of their service. Which? said more needed to be done however.
The tests from Which? also suggested that only 17% of homes were achieving the advertised speed, with even fewer during peak evening times.
The problem seemed particularly bad in rural areas where 98% of homes were unable to get the advertised headline speed of the broadband service they had chosen.
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."
The test also indicated that only 4% of customers on TalkTalks 17 Mbps package were getting the top advertised speed and just 1% of those on BT's 76 Mbps deals.
A spokesman for TalkTalk said: "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."
Meanwhile, BT said in a statement: "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."
Which? would like Ofcom to work with the Advertising Standards Authority to ensure that broadband providers advertise speeds that are actually available for the majority of their customers.
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