Spam levels are at their highest for more than a year, according to the MessageLabs Intelligence: April 2009 Report (PDF), rising to 85 per cent of all email, an increase of more than nine per cent on last month.

MessageLabs attributed much of the rise to a sharp jump in the use of image spam, which uses a .GIF or .JPG file rather than plain text.

The tactic emerged in late 2006 as a way to defeat anti-spam filters, but was thought to have fallen out of favour. MessageLabs believes that spammers are again using the technique to bypass protections.

"These images are now being hosted on what appear to be trustworthy hosting sites, whilst taking advantage of redirection links from reputable sites in order to obfuscate the true location of the image hosting," the company said in the report.

"This is also a technique employed by spammers to evade spam filters that examine the domains of hyperlinks contained in the email."

The latest spike follows a larger trend of rising spam levels over 2009. Worldwide spam levels plummeted late last year after the shutdown of hosting firm McColo, but have recovered this year as spammers migrated to new providers.

Spammers are still taking advantage of breaking news stories, most recently the panic surrounding the Swine Flu outbreak, to flood mailboxes with links to online pharmacies.

In related news, security firm Sophos has removed the UK from its 'Dirty Dozen' spam list for the first time in more than two years. UK systems account for roughly two per cent of all spam sent worldwide.