Only one new virus, Mytob.Z, made it into the top 10 list for April, according to antivirus data from Sophos.

Top of the list was Zafi.B, which accounted for nearly half of all viruses detected. This is the fifth month Zafi.B has topped the charts.

"Although Mytob.Z only accounts for a small percentage of the top 10 reports, it is the only new worm that has managed to break into the stronghold of old threats," said Carole Theriault, security consultant at Sophos.

"First sighted in mid-April, Mytob.Z is a nasty piece of work. Not only does it spread ferociously, but it plants a backdoor Trojan which can be used by remote hackers to gain access and control of a victim's computer.

"The computer can them be spied upon or used to send spam or launch denial-of-service attacks."

Roughly two per cent of all emails contained viruses in April, slightly down on last month.

The Sophos email hoax list was equally quiet. The Hotmail hoax retained its top 10 position and WTC Survivor entered at number six. This claims that a virus is circulating purporting to be information from 11 September.