Millions of internet customers are getting less than half the broadband speed they are paying for, restricting their ability to download music, film and games.

About 60 per cent of broadband customers pay a premium for super-fast packages, but a survey conducted for Ofcom, the telecoms regulator, suggests that a typical household is receiving only 45 per cent of the speed advertised by their internet service provider (ISP).

The research, based on ten million tests in 1,500 homes over 30 days, also confirmed suspicions of a growing digital divide between town and country. People in urban areas enjoy speeds up to 15 per cent higher than those in rural areas.

Internet speeds, using either digital subscriber lines or fibre-optic cables, such as those offered by Virgin Media, also depend on the time of day. Speeds were lowest between 5pm and 6pm on Sundays, when use of the internet is at its peak.

As the number of new internet services, such as high-definition video on demand and internet television, increases, consumers are demanding higher speeds. Many are paying a premium for speeds of up to 8Mb per second. Yet the watchdog found that the average maximum speed was only 4.3Mb. Ofcom said that the speeds achieved were “significantly below advertised headline speeds”.

Publication of the survey provoked more criticism of broadband services, which are lagging behind comparable services in other countries. South Korea, for instance, has built super-fast fibre-optic networks, but in Britain most customers still access the internet via copper lines.

Don Foster, the Liberal Democrat culture, media and sport spokesman, said: “This is yet more evidence that broadband providers are ripping off British consumers.”

Jeremy Hunt, the Shadow Culture Secretary, is expected to voice his criticism today. In a speech to be delivered at Microsoft’s London offices, he will say: “The UK has some of the slowest broadband speeds in the developed world, coming 21st out of 30 countries in a survey by the US-based Information Technology & Innovation Foundation.”

BT has proposed investing £1.5 billion in a fibre-optic network to give 10 million households, or 40 per cent of the population, speeds of 40Mb to 60Mb by 2012. The company has made clear that it will make the move only if Ofcom allows it to make a fair return on its investment.

BT is in a spat with Ofcom over the amount that the regulator will allow it to charge other companies for access to its copper network. BT insiders have suggested that if Ofcom does not raise the prices high enough the company will not invest in fibre optics.

Last month Ofcom published a code which demands that ISPs provide an accurate estimate of the maximum speed. So far 32 ISPs, covering 95 per cent of customers, have signed up.

The Advertising Standards Agency has criticised ISPs for misleading advertising in the past. Three complaints against Tiscali, which claimed that customers would receive up to 8Mb, were upheld by the agency in December, including one by a rival provider, TalkTalk.

In July last year the agency upheld a complaint by BT against Virgin Media that during peak times users would not be able to download songs or TV shows at the speed and times cited in Virgin Media adverts.

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