Those who have fallen victim to "get rich quick" scams or bogus lotteries that cost the public £3.5 billion a year are likely to fall victim again to such schemes, the consumer watchdog the Office of Fair Trading said.

But strangely the average victim is not a bad decision maker and often has had a successful business or professional career according to research conducted for the OFT by the University of Exeter.

It said 20% of Britons are likely to be particularly vulnerable to scams.

People who have a background knowledge of the subject of the scam, such as those already investing directly in shares, are actually often at greater risk because they are over-confident.

Scam victims rarely tell either friends or family that they have been caught out.

Among the commonest tactics used by scammers is to exploit basic emotions like elation or fear to provoke a "gut reaction" from the victim which pulls them into the web.

Mike Haley, director of consumer protection at the OFT, said: "This research provides valuable insight into the sophisticated, heartless and calculating psychological techniques used by scammers to exploit consumers.

"Scams often have a devastating emotional as well as financial impact on their victims."