About a quarter of all spams come as an attachment, according to a study by security supplier Marshal.
While the sophistication of spamming remains low at the moment, if the spammers' social engineering skills improve, Britain could face a security crisis as harmful spam causes havoc, warned Ed Rowley, Marshal's head of technical sales.
"When spam senders improve their social engineering skills and they know enough about their targets to write an enticing or convincing e-mail top line, they could net many more victims," he warned. "This is what is known as spear phishing. They know enough about you to attach a convincing looking document - like an invoice relevant to your business - and you are suckered in."
Marshal reported today that spam containing attached PDF, Excel, Text and Zip files now represents almost 25% of all spam, which has surged up from 2% last week. Meanwhile the previously dominant Image Spam has reached a 12-month low of just 6% of all spam.
"Spam senders have to constantly evolve their forms. It will not be long before they evolve to make their intros look more genuine," he warned.