IT contractors specialising in data security have been warned that the use of spam emails is becoming increasingly malicious.

Spam emails are being used to send Trojan horse infections that can remotely control many thousands of computers and phish for personal details, according to the senior technology consultant for Sophos, Graham Cluley.

It is therefore vital that small businesses and individuals make sure that spam-filtering software is as up-to-date as possible and caution is exercised at all times.

"Furthermore, you should make the pledge never to buy goods advertised via spam, and never respond to spam messages - it just encourages them to send more," Mr Cluley explained.

His comments come after research from Sophos shows that the US remains the main country of origin for spam emails.

The US now generates 18.6 per cent of the spam emails generated, while the UK is responsible for five per cent of the global output, the report claims.