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Google’s email service, Gmail, has seen a two-fold rise in spam in just one month, according to new data from security experts at MessageLabs.

The firm’s report for February indicated that 4.6 per cent of spam now originates from web mail-based services, with the proportion of spam coming from Gmail alone increasing from 1.3 per cent in January to 2.6 per cent the following month.

Yahoo! Mail was the single most targeted application, sending 88.7 per cent of all web mail-based spam.

One of the new technologies to emerge from hackers was the ability to evade spam detection, such as CAPTCHA - a mechanism which is designed to need human input in order to sign in.

“There are several approaches a spammer can take to defeat a CAPTCHA,” said Mark Sunner, chief security analyst, MessageLabs.

“Whether they do so using an algorithm, a ‘mechanical turk’ or combination of the two, email providers are feeling the pressure to keep pace but are limited to what a human can realistically solve creating ever more doubt surrounding the long-term effectiveness of the CAPTCHA as a security mechanism for protecting email services from abuse.”

Spammers finding a way round CAPTCHA is being cited as one reason why Gmail may have seen such a dramatic increase in spam.

Security software company Sophos has revealed that the US relays more spam than any other country in the world.

Websites in the US are responsible for 28.4 per cent of all spam, followed by South Korea with 5.2 per cent and China, including Hong Kong, with 4.9 per cent.

Russia, Brazil and France are the fourth, fifth and sixth worst respectively for relaying spam, the Sophos report revealed.

Carole Theriault, senior security consultant at Sophos, said: ‘It seems as though a major American spammer is arrested every other week at the moment, but despite these high-profile lawbreakers being put away, the US continues to relay far more spam than any other nation on the planet.’

The spamming is not just the work of a few cyber criminals, but represents thousands of people controlling thousands of zombie computers, added Ms Theriault.

Recently, a security report from the Georgia Tech Information Security Centre warned that online videos are the new favoured way for cyber criminals to hide malicious code.