Experts have reported a "disturbing rise" in the level of spam between April and June 2008 as cyber-criminals increasingly exploit Facebook and mobile phones to spread junk mail.

Sophos said that the level of spam rose to 96.5 per cent of all business email last month.

The figure is up on the 92.3 per cent in the first three months of the year, and corporations are now facing the fact that only one in 28 emails is legitimate.

Sophos discovered that spammers are increasingly using networking websites such as Bebo, Facebook and LinkedIn to send links to online stores and bogus lottery and financial scams.

In May, the LinkedIn business networking system was used by scammers seeking to swindle money from unwary corporate executives. On this occasion, the spammers offered a share of a non-existent $6.5m inheritance fund.

"Spammers are finding themselves increasingly obstructed by corporate anti-spam defences at the email gateway," said Graham Cluley, senior technology consultant at Sophos.

"In a nutshell we are stopping the bad guys getting their marketing message in front of their intended audience.

"To get around this, spammers are exploiting networks like Facebook to plant spam messages on other people's profiles which do not just get read by the owner of the profile, but anyone else visiting his or her page."

Sophos identified the US and Russia as the most prolific relayers of spam.