Subliminal messages prompting email users to buy hyped shares have been found in pump-and-dump spam messages, according to security firm SoftScan. 

Pump-and-dump email scams attempt to persuade recipients to buy shares in little known companies on the basis of information 'not known' to the market at large.

Reasons to buy the stock might include an upcoming 'product innovation' or leaked 'secret' merger discussions.

Once enough people have bought the stock to push up its price, the spammer sells their stock, making a healthy profit. Victims are then left holding possibly worthless shares. 

The scam is usually perpetrated without the knowledge of the company involved, causing catastrophic damage to its finances and reputation as its share price spikes and collapses within the space of a day or two.

SoftScan's analysis of the latest pump-and-dump scam has discovered that an image appears for a split second every so often in the email with the word 'buy' repeated several times.

"This latest twist using advertising techniques banned from films in the late 1950s is an interesting development and is aimed purely at encouraging the recipient to buy," said Bo Engelbrechtsen, corporate communications manager at SoftScan.

The security firm warned that, although it appears that this technique is being used mainly for US stocks, it is likely to spread to other countries.