It is the digital dream: an internet service fast enough to download a film in seconds. However, it remains an aspiration, rather than a reality, for millions of online customers as Britain becomes increasingly divided by access to broadband.

Yesterday Virgin Media announced what it claims is Britain's fastest broadband internet service. Its 50Mb per second connection provides nearly nine times the speed of the average broadband service, and can download a television show in a minute, and an entire film in no more time than it takes to listen to a pop song.

The service comes at a price, however: £51 a month, or £35 if users also subscribe to a Virgin phone line for an extra £11 per month.

The actual speeds that customers can expect to achieve may be an entirely different matter, however, as research by Ofcom, the industry watchdog, shows that nearly a quarter of households do not get the broadband speeds for which they have paid.

The vast majority of the country's broadband internet is provided through BT's telephone network. The farther you live from a telephone exchange, the slower the internet service. An 8Mb connection can be as slow as 512kb per second if you are five miles from an exchange. It would cost as much as £29 billion for BT to upgrade the whole country from copper wires to super-fast fibre-optic lines, a recent report said.

Virgin Media's cable connection does not suffer from that problem, but the service will be available to only about half of all households by next summer.

At the moment it is accessible to 1.3million homes spread around Britain in places including Dundee, Bradford, Liverpool, Croydon and Wolverhampton.

Analysts said that Virgin's so-called super-fast service would be dramatically slowed if customer numbers grew. Ian Fogg, principal analyst at Forrester Research, said that customers who signed up should not expect to obtain the fastest advertised speeds.

He said: “The reality will be a bit different. How successful it will be is how successful Virgin Media will be with customers. The more customers that sign up, the lower the speeds customers will actually receive, because of the way the network is structured.”

Virgin Media insisted that extensive trials on the network showed that customers would get speeds of at least 45Mb 80 per cent of the time.

Neil Berkett, chief executive of the company, said that the service marked a revolution in the use of the internet. “We're not just launching a product, we are catalysing a step change in the UK's development,” he said.

The announcement came as Kenneth Branagh, Richard Curtis and a number of British film and television directors and producers called in a letter to The Times for internet providers to do more to tackle illegal downloading.

The group, voicing concern that “the successes of the creative industries in the UK are being undermined by the illegal online file-sharing”, said that internet providers should be compelled by law “to change the behaviour of those customers who illegally distribute content online”.

Callum McDougall, the producer of the James Bond film Quantum of Solace, said: “Film-makers rely on DVD and download sales to recoup the costs of making a film. There should be a law forcing internet providers to warn customers when they download material illegally.”