A new global botnet menace dubbed Kraken is taking over from the once huge Storm malware network, experts warn.

Dave Marcus, security research and communications manager at McAfee, told vnunet.com that the company had noticed a drop in Storm activity which he attributed to the rise of Kraken.

The new botnet has usurped Storm as the largest on the web, and Marcus suggests that even Storm's operators are moving on to the new piece of malware for their operations.

"The bad guys are clever, and this Kraken piece of coding has definitely learned from Storm," he said.

The Storm botnet has shrunk to a twentieth of its previous size, according to security firm MessageLabs.

Previously estimated to encompass some two million compromised computers, the total number of PCs under Storm's control now sits around 100,000, the company claims.

Along with the drop in botnet muscle, Storm's activities have also slowed. The number of spam emails MessageLabs traced back to Storm fell by 53 per cent in April.

Storm had spent more than a year as the largest malware botnet on the web and was a major threat to users.

MessageLabs credits the April drop to advances in security software enabling users to detect and remove the Storm malware.

Users should not, however, write off Storm just yet, warned Marcus. The botnet that dominated the cyber-criminal world for more than a year could very well make a comeback.

"I do not think Storm will go away," he said. "It may lull for a while, but you will see it pick up again."