BT released figures today which showed that the number of attempts to access websites hosting child pornography have trebled in the past 18 months.

BT reports that it is now blocking 35,000 attempts to access child pornography in Britain every day.

And one leading e-commerce figure has described over Internet Service Providers' failure to block access to such sites as "outrageous", and called on other ISPs to follow BT's lead.

The figures from the telecoms company, which accounts for one-third of the internet market in the UK, are based on data acquired since it introduced a filter in June 2004 to block access to child porn sites.

It is feared the overall number could be much higher and that many attempts to access illegal material are slipping through filtering technology.

"While it’s good to see that BT has taken steps to stop its users from being able to view illegal child pornography, clearly the message that not blocking users from viewing this type of material isn’t acceptable isn’t getting through to other ISPs," Mark Herbert, the founder of intY, the business internet and e-mail specialists, told Times Online today.

"It’s outrageous that ISPs allow their customers to access this type of illegal material, they shouldn’t be exposed to it even accidentally through an innocent Internet search.

The industry has shifted responsibility on to the individual or business to stop people accessing these sites using Internet access controls. If one ISP can put controls in place then surely others can as well?" Mr Herbert said.

The Cleanfeed program, which prevents users from accessing sites blacklisted by the Internet Watch Foundation (IWF), initially registered 10,000 attempts to access them by BT’s 3.1 million domestic users each day.

Today’s figures showed there were 4 million blocked attempts to access the websites over the last four months. Downloading images from such sites could constitute an illegal act which is punishable with a jail sentence.

The report will alarm policy makers and the internet industry, which have sought to control child pornography through a combination of approaches.

Members of the public and computer repair shops play an important part by reporting to the police people they think could possess or be distributing illegal images. The Internet Watch Foundation, www.iwf.org.uk, also investigates suspicious websites.

The British authorities have also taken part in international operations aimed at tackling the international market in illegal images. In the most high-profile clampdown, Operation Ore, the American and British authorities co-operated to catch people downloading child pornography using credit card details and information from internet service providers to track down offenders.

In 1997, when IWF came into operation, 18 per cent of the potentially illegal content assessed by the body - almost exclusively child abuse images - was hosted in the UK. At the end of 2003, that figures had fallen to less than 1 per cent.

The Home Office has consistently pointed out that Britain already co-operates closely with other countries over how the internet is policed and says this approach will continue. However, tracking down the source of images available in the UK, which could come from almost anywhere in the world, can still be very difficult.

There has also been resistance to surveillance of online behaviour from some British organisations. Internet Freedom, the campaign group, last year warmed that fresh measures proposed by the Government to criminalise the possession of obscene material distributed online risked limiting people’s freedom.

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