British police are working with online bookmakers to head off blackmailers threatening denial of service (Dos) attacks.

With bookmakers expecting £50m to be staked on the Grand National this weekend, online gambling firms are on alert for attempts to bring down their sites.

A spokeswoman for the National Hi-Tech Crime Unit (NHTCU) told vnunet.com: "Investigations have been going on since last September. The NHTCU has been working with UK online gambling organisations and will continue to do so."

Dos attacks flood servers with information requests, often using a network of 'zombie' PCs controlled after being infected with viruses.

Downtime for websites during top races could see bookmakers missing out on millions of pounds in turnover.

"This is more likely to be a bunch of smart kids in Taiwan than any organised criminals," said David Hook, head of public relations at bookmaker William Hill.

"Criminals would be more organised and efficient. Today we know of no-one who has paid [the blackmailers], and no-one that has been attacked more than once."

Hook confirmed that the William Hill website had been hit with a Dos attack, but claimed that it resulted only in the site running a little slower than normal.

Antonia Sharpe, corporate public relations manager at Ladbrokes Worldwide, said: "We are better protected than other sites because of our high bandwidth. We made the investment when we set it up and it has reaped benefits.

"There is a lot of co-operation between the industry and the authorities over this kind of thing, but you should remember that online gambling only accounts for a fraction of our revenue."

National Hi-Tech Crime Unit