The US relays more spam than any other country, according to malware specialists Sophos.

Nearly a quarter of all spam was sent from computers in the US during the past six months, compared to just over 20 per cent from China and Hong Kong. The UK was tenth in the list, contributing 1.9 per cent of the world's spam.

"We are seeing spammers being successfully brought to justice in the US, but it's important to remember that they can be based anywhere in the world," said Graham Cluley, senior technology consultant at Sophos.

"In fact, the vast majority of spam is generated from zombie computers, hijacked PCs infected by malware. A combination of technology, legal action, user education and international co-operation is needed to truly make spam a thing of the past."

Poland, Spain and Germany all saw their spam-relaying percentages rise between the last quarter of 2005 and the first of 2006, while the UK, which was absent from the previous 'dirty dozen', has now re-entered the table in tenth position.

These changes have helped Europe challenge North America for second position in the top spam-relaying continents. Asia takes the top spot at 42 per cent of the world's spam.