Millions of broadband customers are being sold super-fast connections that their internet providers are unable to deliver, according to new research.

Data released by Ofcom, the communications watchdog, shows that the average broadband speed is still less than half of the average advertised speed.

The report, published on Wednesday, says that Britons get an average broadband speed of 6.2 megabits per second (Mbps) – less than half the average advertised speed of 13.8Mb.

Ofcom is pushing for a change in the way internet providers, including BT, Sky and O2, advertise "up to" broadband speeds, which most customers are unable to receive.

Its findings will feed into a consultation now underway by the Advertising Standards Authority's committee of advertising practice (CAP) and broadcast committee of adverting practice (BCAP) into how broadband speeds are advertised. That is expected to report in the next three months.

"The research shows that ISPs need to do more to ensure they are giving customers clear and accurate information about the services they provide and the factors that may affect the actual speeds customers will receive," said Ed Richards, Ofcom's chief executive.

"It is important that the rules around broadband advertising change, so that consumers are able to make more informed decisions based on the adverts they see, and that advertisers are able to communicate more clearly how their products compare to others in the market."

Ofcom carried out performance tests in 1,700 homes across the UK in November and December last year. The report is based on 11 broadband packages from the UK's seven largest providers, including Virgin Media and TalkTalk.

The growing demand for faster broadband packages has led ISPs to advertise maximum speeds, despite current infrastructure being unable to support them. The only ISP delivering close to the maximum speed advertised, according to Ofcom, was Virgin Media, with the advantage of a relatively new cable network in many urban areas.

The typical broadband speed received by customers was much closer to the advertised speed with fibre-optic cable packages, such as the those delivered by Virgin Media and BT's Infinity package. However, only 22% of Britons have fibre-optic connections, with 77% of the population using copper-based DSL phone lines.

The greatest disparity in advertised and delivered broadband speeds came with "up to" 20Mbps connections delivered over the current-generation DSL lines. Only 3% of customers with this package received an average speed close to 20Mbps, with 69% receiving an average speed of less than 8Mbps.

The overall average broadband speed increased from 5.2Mbps in May last year to 6.2Mbps in December, the report shows, meaning a typical five-minute song would take just under four seconds to download, compared with just over five seconds on the slower speed.

Ofcom recommends that broadband speeds should be advertised on a "typical speeds range" – the speed which customers can expect to receive. The regulator says typical speeds must "have at least equal prominence" to headline speeds, and that maximum speeds must only be advertised "if it is actually achievable in practice by a material number of consumers".

Under Ofcom's recommendations, ISPs advertising DSL connections of "up to" 8Mbps – the service used by the majority of Britons – would be forced to reveal that the typical speed is in reality between 2Mbps and 5Mbps.

However, BT said it had "real concerns" with Ofcom's approach. "Moving to typical speed ranges will potentially be highly misleading as the average performance will vary depending on where people live," said John Petter, the managing director BT Retail's consumer business. "Enforcing typical speed ranges is also dangerous as it could encourage more ISPs to cherry pick customers who will increase their average, leaving customers in rural and suburban areas under-served. That would encourage digital exclusion rather than tackle it."

Sky said the debate about headline speeds had the potential to be a "red herring", given that existing Ofcom guidelines state that providers must give customers individual speed estimates at the point of sale.

Jon James, executive director of broadband at Virgin Media, described the report as "yet another damning indictment that consumers continue to be treated like mugs and misled by ISPs that simply cannot deliver on their advertised speed claims".