Ofcom data shows average speeds in six months to May were 6.8Mbps compared with average advertised speed of 15Mbps

Millions of Britons are still being sold "superfast" connections that are only half as fast as advertised, according to research by Ofcom.

Data released by the communications regulator shows that although Britons benefited from a 10% increase in average broadband speeds around the UK in the six months to May, reaching 6.8 megabits per second (Mbps) compared with 6.2Mbps in November 2010, the average advertised speed was 15Mbps.

The data emerged from a survey of the home connections of 1,700 people.

The greatest disparity in advertised and received broadband speeds came with copper-based DSL phone lines, used in more than 75% of UK homes. Customers with broadband packages offering speeds "up to" 20Mbps and 24Mbps actually received an average speed of 6.6Mbps, according to the research – and more than a third of these customers got average speeds of 4Mbps or less.

Ofcom has long pushed for a change to the way internet providers, including BT and Sky, advertise "up to" broadband speeds that most customers are unable to receive.

Ed Richards, the chief executive of Ofcom, said: "We are now seeing consumers increasingly move to higher-rated services and enjoying genuinely faster speeds. Consumers also have access to better broadband information, allowing them to decide which provider to use based on actual speeds they can achieve at home.

"However, the research is still telling us that some consumers are not receiving anywhere near the speeds that are being advertised by some ISPs. Ofcom continues to urge the CAP and BCAP committees to make changes to their advertising guidance so that consumers are able to make more informed decisions based on the adverts they see."

The Advertising Standards Authority's committee of advertising practice (CAP) and broadcast committee of adverting practice (BCAP) are expected to report in the early autumn on changes to the way broadband speeds are advertised.

Four of the largest ISPs – BT, Sky, Virgin Media and O2 – have signed up to Ofcom's new code of practice, which comes into effect on Wednesday and dictates that customers must be given an expected speed at the point of sale.

The growing demand for faster packages – fuelled by the popularity of traffic-intensive video services such as the BBC iPlayer and YouTube – has put current infrastructure under huge strain, meaning most ISPs are unable to deliver advertised speeds, especially at peak periods when large numbers of people use the services simultaneously.

Customers with fibre-optic connections get speeds much closer to those advertised, such as the those from Virgin Media and BT's Infinity package. The report shoes that the average download speed on Virgin Media's 30Mbps package was actually marginally higher than the advertised speed.

Only about 22% of Britons have fibre-optic connections, despite them being available to 57% of UK homes.