Thousands of British government, National Health Service, school and police Web sites have been doctored to include links to pornography, viruses and other inappropriate material.

An investigation by The Times and Trend Micro, the Internet security specialists, has revealed that a large number of taxpayer-funded official Web sites have been hit by a practice known as "link spamming."

This is when a hacker, or a member of the public, adds links to an official Web site, which then point visitors to other, inappropriate, material.

It means that an unsuspecting parent might click on an innocent-looking link on his child's school site and be taken to a pornographic site instead, or a patient might click on an NHS Web site link only to download a harmful computer virus without her knowledge. Many government-created Web sites are potentially unsafe.

The practice has become widespread, affecting the pages of Whitehall departments such as the Department of Education, fire brigades, police forces, local and district councils, NHS and health service sites and a large number of schools across the country.

Experts said the findings exposed the poor security of government-run sites, which has left them open to abuse. Sometimes a hacker will illegally infiltrate a site to add links that are then hidden from view.

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