Some 40 per cent of British Internet users say they have accidentally come across pornography while surfing, according to a survey by the British Consumers' Association.
It said parents among 7,000 people surveyed reported being shocked to come across pornographic Web sites, sometimes when browsing with their children.
The association said it was working with search engines to crack down on porn sites.
A separate survey showed the number of adults with the Internet at home slipped to 39 per cent in August from 40 per cent three months earlier.
The Office of Telecommunications (OFTEL) said the survey, covering over 2,000 people, reversed an 18-month upward trend.
OFTEL officials said it was too early to speculate on the reason for the fall, and the Internet market as a whole was broadly unchanged.
However when access at work or in Internet cafes is included, the figure rises to 49 per cent.
Cumulative results showed home Internet access has risen steadily from about 10 per cent in January 1999.
OFTEL, which conducts surveys to help in policy-making, said it was the first time it had asked questions about alternative ways to access the Internet.