Companies are being advised to ensure their systems are protected against denial-of-service (DoS) attacks, as the problem of online blackmail is growing.

According to Duncan Hine, business centre director at Qinetiq - formerly the government's Defence Evaluation and Research Agency - attacks on company systems are becoming more professional, and now often involve organised gangs.

"When blackmail is linked to a denial-of-service attack you know that (the criminals) will have a real source of professional expertise behind them," he added.

Last year, many online bookmakers found themselves targeted by blackmailers who threatened to disrupt web sites by launching denial-of-service attacks.

Hine said the number of blackmail threats is growing steadily. In response, firms should establish procedures for coping with threats and attacks in advance.

"Companies that have not prepared face a problem," Hine warned. "There is real danger in reacting on the hoof," he added.

Furthermore, firms should not assume that only well-known organisations will be targeted.

"The targets will not be the larger firms as they have resilient hosting and counter-measures in place. This is not the case for smaller firms," warned Hine.