Up to three-quarters of households are paying for advertised broadband speeds that they have never received, a survey suggests.
Consumer champions Which? found that 74% are not receiving the promised headline speeds on their broadband packages - the equivalent of 15.4 million homes - despite nine in 10 people saying this was an important factor when choosing a provider.
The watchdog also found that just 17% of homes received an average speed that matched the advertised level, while even less - 15% - managed this during the peak evening period.
The study also found that "incredibly" 98% of rural homes did not typically receive the advertised headline speed, while 31% of households in towns and cities were able to receive the maximum level.
Advertising guidelines say speeds quoted in broadband adverts need apply to a minimum of 10% of all customers.
But Which? said it found three packages that could not meet the minimum requirement, with only 4% of customers on TalkTalk's 17Mbps package and 1% of those on BT and Plusnet's 76Mbps deals receiving the top advertised speeds.
Ofcom announced changes last week designed to make it easier for customers to switch their broadband provider if they are not receiving the speed advertised on their contract.
But Which? said it wanted the rules changed so that providers are only allowed to advertise speeds that most of their customers can receive.
Which? executive director Richard Lloyd said: "It's 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.
"Broadband is an essential part of life these days so people shouldn't be persuaded to buy a package which is never going to live up to expectations.
"We've raised our concerns with the advertising authorities, but we now want Ofcom to ensure consumers get the speeds promised by providers."
:: Populus surveyed 2,339 UK adults online on September 17-18 2014.
A TalkTalk spokesman 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."
A BT spokesman said: "BT uses the method to describe our speeds that is defined by the Committee of Advertising Practice and followed by all internet service providers.
"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."
An Ofcom spokeswoman said: "It is essential that people know exactly what to expect when they buy a broadband service.
"Ofcom's new code ensures people will be given clear, accurate information about their broadband speeds before they sign up. And customers have the right to walk away, without penalty, if speeds fall below an acceptable level."
Shadow culture secretary Chris Bryant said: "Under this government people are getting ripped off by companies being allowed to advertise speeds they simply don't deliver. We need an end to Government inaction and for ministers to start standing up for consumers.
"People rely on broadband as an essential part of everyday life but it's on this Government's watch that taxpayers' money is being used to fund a broadband rollout that's not meeting the needs of most communities and customers.
"We need transparency about the real speeds available to most customers, proper regulation of advertising and real action to deliver superfast broadband to the whole of the UK."
A spokesperson for the Department of Culture, Media and Sport said: "Consumers need accurate information on broadband speeds to make informed purchasing decisions. Ofcom and the Committee of Advertising Practice provide the rules and guidance on advertising broadband speeds and will be considering these concerns.
"The government's rollout of superfast broadband is now reaching 5,000 more homes and businesses every day, and on track for 95 per cent of the UK having access to speeds of at least 24Mbps by 2017."
A Plusnet spokeswoman said: "Plusnet takes adhering to industry standards and regulations extremely seriously. We meet the standards set by the Advertising Standards Authority and Committee of Advertising Practice and follow the Ofcom code of practice. When any customer is thinking of joining us we take them through the speed range for their individual line and clearly display this.
"Ofcom's latest broadband performance report highlights our ADSL2+ speeds are also consistently faster than our competitors on a 24-hour average and at peak times when it matters most to our customers."
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