Broadband customers are being sold super-fast connections that their providers are unable to achieve, according to research that shows internet users are struggling with speeds of less than half the rate they signed up to.

The media and telecoms watchdog, Ofcom, examined the most popular broadband product advertised as offering speeds of "up to" 8 megabits per second and used by 57% of homes. The report, published today, says, on average, users are getting speeds of just 3.9Mb. That means a DVD-quality film would take more than two hours to download – longer than it takes to watch it – compared to just over an hour at the faster speed.

Fewer than one in 10 households on an up to 8Mb service get over 6Mb and no one can ever receive the full speed because of the way the internet service providers (ISPs) run their networks, the report said. The average speed for all broadband connections in the UK, according to Ofcom, is just 4.1Mb. That doubles the time it takes to download a music track – 10 seconds as opposed to 5 seconds at the fast rate.

The lower speeds have led to a flurry of complaints from consumers. More than one in five of those surveyed by Ofcom expressed dissatisfaction with the speed they are getting while 26% said they did not get the speed they expected to receive when they signed up.

The research comes after Gordon Brown recently said broadband internet access was as important to British homes as gas and electricity. It also shows a wide disparity in the service from individual service providers: Tiscali and AOL, both owned by Carphone Warehouse's TalkTalk, scored lowest on speed while Virgin Media's comparable up to 10Mb service was the fastest, not least because it uses different technology and its network only reaches half the country.

The survey, compiled by technology experts SamKnows and market research firm GfK, tracked the internet connections of more than 1,600 users over six months in a process that involved 60m readings. It also proves what many consumers have been complaining about for some time: during peak times internet connections are being slowed down. Between 8pm and 10pm when many people are using video services such as the BBC's iPlayer, the average speed of a top-of-the-range service slows by more than half.

Consumer groups have called for widespread changes to the advertising of broadband connections. "There are some real questions to ask around the transparency of advertising in this industry," said Matt Bath, technology editor at Which?.

The Communications Consumer Panel, which advises Ofcom on consumer issues, has been pushing for advertising using claims of up-to speeds to be scrapped in favour of average speed, but has been frustrated by the lack of reliable data. Its chair, Anna Bradley, believes the new Ofcom research could provide a fresh benchmark that will give consumers a much better idea of what they can expect before they sign on the dotted line.

"The Ofcom work is incredibly helpful because it exposes just how little the up- to speeds mean and exposes how variable the service level is from one provider to another," she said. "The fact that they are still advertised with 'up-to' speeds is deeply problematic."

But Ofcom's Peter Phillips, said ISPs "are not lying to consumers. If you are very close to the exchange you can get those speeds."

Late last year the industry agreed a voluntary code of practice with Ofcom which has been signed by 50 of the UK's biggest ISPs, accounting for 95% of all internet connections.

It requires them to tell customers the maximum speed they can expect from their line, as speed is affected by distance from the local telephone exchange, and allow them to trade down if they cannot get the service they want. Ofcom is currently undertaking a so-called "mystery shopper" exercise to ensure the code is working, but if consumers are being mis-sold, Ofcom could introduce a mandatory code of practice which the whole industry would be required to abide by and would include penalties.

BT has already attacked the report as "unreliable" because it used too few people, and as out of date because the company is migrating to new broadband technology. TalkTalk, meanwhile, has pointed out that it is moving Tiscali customers on to its own network, which should improve their service. A Virgin Media spokesman said: "It's what customers get, not what their ISP claims, that counts."