Prime Minister Tony Blair told MPs yesterday that a government-backed summit to work on preventing the dissemination of child pornography on the internet will be held in September.

Internet service providers will be invited to share their approaches to preventing access to sites containing illegal material.

"We welcome the BT initiative to block access to web pages containing images of child abuse," Blair told Parliament. "We would encourage any measure that other ISPs take to reduce the availability of such images."

BT claimed earlier this week that it had prevented almost 250,000 attempts to access child porn websites since it switched on its Cleanfeed censoring system on 21 June.

The system blocks access to several thousand websites on a blacklist compiled by UK industry body the Internet Watch Foundation.

But another UK trade body, the Internet Service Providers' Association (ISPA), has expressed concerns over the reporting of figures from BT.

"ISPA welcomes new developments in the fight against child abuse images appearing on the internet," the organisation said.

"However, ISPA feels that caution is needed with the information and statistics so far available on Cleanfeed. At present there seems to be a significant disparity in the statistics being reported."

ISPA also pointed out that it is unclear whether Cleanfeed is measuring the number of visits to these illegal websites.

Some of the traffic is likely to have been generated automatically by malware on infected computers without the users' knowledge. But BT has so far been reluctant to conduct such analysis.

In 2003 less than one per cent of illegal images reported to the Internet Watch Foundation were hosted on UK-based sites.