Experts are worried about the security protection on Gmail after a recent rise in spam volumes.

Researchers at MessageLabs found that spam messages originating from Google's webmail service doubled last month to reach roughly 2.6 per cent of all webmail spam.

MessageLabs believes that this points to a possible breach of Gmail's spam protections, in particular the 'Captcha' system.

Captcha codes are the input boxes in which a user copies a sequence of letters or numbers from an image. The system is designed to prevent spammers registering multiple accounts automatically.

"There are several approaches a spammer can take to defeat a Captcha," said Mark Sunner, chief security analyst at 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.

"This is creating ever more doubt about the long-term effectiveness of Captcha as a security mechanism for email services."

A Google spokesperson declined to comment on the MessageLabs report, but did tell vnunet.com that the company has taken action against the spammers.

"Fighting spam is a never-ending battle," said the spokesperson. "We disabled these accounts immediately and will continue to do so if they spread."

MessageLabs acknowledged that Gmail is a relatively minor source of spam. While the February boost brought Gmail's total to 2.6 per cent of webmail spam, Yahoo Mail accounts for a whopping 88.7 per cent.