The NOP poll shows that one in five of nearly 4,000 children between the ages of six and 16 interviewed for the survey between September and October this year were "uncomfortable" with some content viewed on-line.

In the UK, 2.4 million children are estimated to use the Internet - roughly a third of all children between six and 16.

Of those who have had negative experiences while surfing the Internet, the largest proportion - 40% - had seen something "rude".

'Not surprised'

One in seven said they had encountered content that had "freightened them", while 25% saw pages that they thought "would get them into trouble".

NOP Associate Director Rob Lawson described the numbers as a "significant minority".

The children's charity NCH Action for Children suggested the survey strengthened calls for Internet regulation to protect younger users.

Charity spokesman John Carr said: "I regret to say I'm not surprised by this survey's findings, it's what we have been saying for some time.

Net nannies

"Parents need to know their children are surfing the net in safety and security. At the moment, they have no way of knowing that at all."

NCH Action for Children, which advises the government on children's issues, backs the introduction of "net nannies" - programmes which filter out content unsuitable for children.

The survey, called Kids.net, was paid for by Microsoft, the BBC, NatWest and Anglia Multimedia in syndicate.