Online security firm Sophos found that the US was responsible for 19.6% of spam sent to inboxes during the second quarter of the year, down from 0.2% on the first three months of 2007.

Europe has six countries in the top 12 spamming nations, which added together, are responsible for more spam than the US.

Overall global spam volumes rose by around 9% in this period, compared to the second quarter of 2006.

The UK and Italy were in joint 10th position in the table, each responsible for sending out 2.8% of spam e-mails.

A Sophos spokeswoman said:

"A consolidated effort is needed not only to pursue and prosecute spammers, but also to convince computer users everywhere of the importance of blocking rather than responding to spam messages. Everyone has a part to play if we are to win the global battle against spam."