The average size of a spam message has increased by 77 per cent since September last year and continues to grow, according to SoftScan. 

The security firm attributes the increase to the dramatic rise in image-based spam in recent months.

SoftScan warned that this will add to the cost managing email for organisations that have to scale-up bandwidth and storage requirements to meet demand.

Since September last year individual spam emails have increased from an average of 6.62Kb to 11.76Kb.

Although still relatively small in size, the sheer volume of spam that many businesses receive means that only a slight rise can have a significant impact.

Organisations that stop spam at their email servers still have to pay for the bandwidth to receive it and, depending on how their email back-up is configured, storage costs may rise if spam is included in the archive.

"Spam is no longer just an issue of user productivity. The growth in file size combined with the increasing volume mean that network administration and internet bandwidth are affected," said Diego d'Ambra, chief technology officer at SoftScan.

"Email file size is going to become a real headache for businesses, particularly if spammers start to use other media such as audio or video files once the tactic of image spam no longer works against the majority of filters."

Spammers use images for a variety of reasons, from avoiding detection to making their products more attractive to potential buyers.

The first junk emails that used images were just graphics attached to an email, but this was soon followed by embedded coloured backgrounds.

More recently the backgrounds have changed to variety of colours in the hope that they will fool scanning techniques.

Image spam is most frequently used with 'pump and dump' spam which tries to tempt recipients to buy shares in the knowledge of a 'hot tip'.

When enough people have bought the shares, thereby inflating the price, the spammer sells at a profit and the price collapses.