Tony Blair yesterday announced a government-backed summit into ways to stop paedophiles accessing child porn on the internet.

The prime minister invited internet firms to the talks in September in a new bid to block online viewing of youngsters being abused.

He also welcomed a BT initiative which used new technology to block access to child porn - but revealed the scale of the problem with 200,000 attempts to view hardcore websites in just three weeks.

Another survey has revealed that more than half of children who surf the internet at least once a week have come into contact with adult pornography online.

Mr Blair urged other firms to follow BT's lead.

"We would encourage any measure which other internet service providers can take to reduce the availability of such images," he told the Commons.

"And we will be inviting leading service providers to a meeting in September to consider what can be learned from the experience of the BT initiative and how that can be extended across the piece.

"This is a serious issue of public concern and we are acting on it...this despicable trade in children's misery", he added.

The summit announcement came as parents were warned to be on their guard following the first detailed national survey into children's contact with online porn revealed worrying results.

Researchers who interviewed more than 1,500 nine to 19-year-olds found that 57% had come into contact with pornography, mainly adult orientated, and a third had received unwanted sexual or nasty comments - including bullying. Only one in 20 of the 906 parents also interviewed appeared to be aware of this.

Parent's ignorance of the dangers their children are facing were also highlighted in the poll, with only 16% reporting that their kids had seen porn online, according to the study by the London School of Economics and Political Science.

It also found that 46% of children interviewed said they had given out personal information while only 5% of the parents were aware of this.

Sonia Livingstone, professor of social psychology at the LSE, and co-author of the report warned that parents need to be more aware of the risks.

"Parents are used to medium that are pretty heavily regulated, with age regulations and a lot of care," she said. "Suddenly the Government is saying this is the medium (the internet) that your children need to get ahead educationally...but there are serious risks."

John Carr, internet adviser to the children's charity NCH said: "The gap between what children are actually doing and what their parents think they are doing is a lot larger than many people would have imagined.

"It is a gap we must try to close."

Stephen Carrick-Davies, chief executive of the children's and internet charity Childnet International, said: "Parents need more practical advice and guidance in getting the most out of the internet."

Around 90% of children currently use the internet for homework, 72% for email and 70% for games; 74% have access to the internet at home and 98% have access somewhere, and 19% have the internet in their bedroom.

Karen Thomson, the chief executive of AOL UK, a co-sponsor of the report, added: "I am concerned that over half of the children interviewed say they have come into contact with pornography on the web and that many have given out personal information, yet with some simple steps and the right software, it is possible to substantially reduce the risks."

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