PDF spam now accounts for a quarter of all spam, according to the latest research statistics from security vendor Marshal.

The news will come as no surprise to many businesses that have seen the fast rise in image-based spam, but its early timing might.

Two weeks ago, the figure was estimated to have reached around 15 per cent of all spam.

The type of attachments causing the most problems are Excel files, PDFs, text and zip files, the first of which has been cited as one of the latest crazes among spammers.

Last week, spam-using attachments only represented 2 per cent of the problem, but this week that figure rocketed to 25 per cent. Marshal claimed that "one core group of spammers" behind the Storm worm were responsible for this week's surge.

"The latest statistics would suggest that spammers are increasingly favouring PDF spam over image spam," said Bradley Antsis, director of product management at Marshal. "However the spammers' experimentation with a range of attachments indicates they are not as confident PDF spam is as effective as image spam was in the past."

Other research conducted recently has indicated that up to half of the UK's medium to large sized businesses are genuinely struggling to deal with image-based spam. Thursday it was revealed that scammers are also using the success of The Simpsons Movie to trick email users into validating their addresses.