The number of attempts to view illegal child pornography on the web has risen sharply since 2004, according to BT.
BT began using Cleanfeed in 2004
The company uses a system to block sites carrying indecent images of children, which has been thwarting 35,000 hits a day for four months.
When BT first began using the Cleanfeed system 18 months ago, there were 10,000 attempted hits every day.
Children's groups have repeated calls for all internet service providers to prevent access to illegal websites.
The Cleanfeed blocking technology forbids access to sites blacklisted by the Internet Watch Foundation, which monitors illegal activity on the web.
Most of the bigger service providers use similar blocking technology to BT, but there is continuing pressure on all UK providers to follow suit.
The foundation blacklisted more than 6,000 websites in 2005 - up from 3,500 in the previous year.
The IWF assesses whether websites are "illegal to view" under the 1978 Child Protection Act.
John Carr, an internet adviser to the children's charity NCH, has welcomed the work being done by BT and other companies.
BT has proven that the technology works so there really isn't any excuse for any internet service provider not doing it
However, he believes the current system of self-regulation is "reaching its outer limits".
He told BBC Radio Five Live that "unless the industry can show pretty quickly that they're at or close to 100% coverage in Britain, I'm afraid we will be going to talk to our MPs... demanding legislation".
He said that as BT estimated it had one-third of the market, to extrapolate across the whole domestic internet market would mean there were 100,000 hits per day or 4 million per year.
According to the industry about 80% of domestic internet users in the UK are covered by BT Cleanfeed-type solutions, he said, but he also raised concerns that there was "some scepticism" about that figure and called for it to be independently verified.
"Even if it's true, it means that one in five domestic internet users could access these illegal sites without any obstacles at all.
"That is not acceptable. It needs to be 100%.
"BT has proven that the technology works so there really isn't any excuse for any internet service provider not doing it," Mr Carr added.