British police and Irish-based internet security specialists have warned businesses to be vigilant this weekend as computer hackers plan to disfigure thousands of websites.

A worldwide "hacking" competition will begin tomorrow, which challenges computer hackers, sometimes known as "scriptkiddies", to deface thousands of corporate websites across the world.

Internet companies said bigger firms, including banks and financial institutions, should have software in place to repel the attack.

But Mr Andy Harbison, head of computer forensics with Ernst & Young in Dublin, said small firms could be affected by the activities of the hackers and should ensure they had the latest security upgrades in place.

He said the hackers would try to sidestep security to change or replace the images that appear on corporate or public websites.

During the competition, each hacker would be awarded points depending on the type of website she or he defaced, said Mr Harbison. Participants were unlikely to be specialists and more likely to be young scriptkiddies, he added.

The British Police National High-Tech Crime Unit said it had warned industry and alerted the computer crime units attached to each UK force.

A spokeswoman said hacking from Britain was a breach of the law under the Computer Misuse Act. But she added that UK authorities were powerless to act against people who hacked into British websites from abroad.

A website, in broken English, sets out the rules for the hackers who take part in the contest. It reads: "Who is the best one of all? Who could participate of this challenge and leave victorious person? This only depends you defacer!"

It adds: "The time of the challenge will have duration maximum of six hours. The challenge. . . for day 6 of July hours yet not defined but probably will be morning."

The prize is said to be Web space and an internet address or domain name. - (Additional reporting, PA)