About 1.5 million adults have stumbled across child pornography while browsing online, an internet watchdog said.

A survey by the Internet Watch Foundation (IWF), set up in 1996 as a UK hotline to report criminal content online, discovered that 4% of men and 2% of women have come into contact with images of child abuse.

However, the survey showed around 40% of Britons would not know how to report child porn if they were to encounter it.

Child porn emerged as the number one concern among adults when considering a range of illegal or harmful content online, with terrorist websites and extreme or violent pornography coming closely behind.

Claire Lilley, from the NSPCC, said: "Recent research we undertook showed that 26 million indecent images of children were confiscated by just five police forces in the last two years. In 1990 it was estimated there were only 7,000 of these pictures in circulation so we can see this vile trade is growing at a disturbing rate.

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"Although industry is working hard to clamp down on this problem, urgent action is needed. Children - many of them too young to go to school - are being seriously sexually abused to make these images which are a stain on our society."

Around 77% of those surveyed were most concerned by child porn, while 73% were worried about terrorist websites and 68% were concerned by extreme and violent porn.

Just under two-thirds were concerned with hate websites, which display racist or homophobic material, 61% are worried about suicide content and around half of those surveyed were concerned by eating disorder websites.

The IWF said the survey revealed that women were more concerned than men across all categories of material.

IWF chief executive Susie Hargreaves said:

"There is clear public concern over the availability of images and videos of children being sexually abused on the internet.

"What is concerning for us is that not enough people know how to report this or would rather ignore it, especially considering the survey tells us that around 1.5 million British adults have seen this sort of content online.

"Although we've seen record removal times in the UK, during 2012 we saw a higher proportion of images of children under 10 years old being sexually abused."

Around 16% of men said they would just ignore child porn if they stumbled across it, the survey said.

In the whole of 2012, the IWF logged just 73 UK websites hosting child sexual abuse images or videos, compared with 9,477 hosted in other countries around the world.

Of the 73 UK webpages, 41 or 56% were then removed within 60 minutes of the IWF notifying the host company or internet service provider (ISP).

However, Ms Hargreaves added: "We are also very aware that there are internet hosting companies in the UK which could do more, and faster, who are not members of IWF.

"We have a responsibility to do all we can to help protect children - and adults who were abused as children - from having their abuse viewed time and time again. We need to prevent people from stumbling upon this content and assist other countries in creating a hostile environment for hosting it."

The ComRes poll surveyed a sample of 2,058 British adults.