See also: How to spot fake currency on holiday
See also: Sharp rise in council tax rebate scams
How does it work?
"Binary options" investment websites allow users to bet on whether shares or currencies will rise or fall in value - often offering impressive returns to those who get it right.
They are binary because there are only two possible outcomes: win or lose. This can make them seem a simple way to make money.
Investing this way is high risk, though. And even legitimate "binary options" sites are unregulated.
Fraudulent versions of the sites, set up with the sole aim of stealing investors' money, are also rife.
They generally lure people in with promises of huge returns, claiming that there is little risk involved.
Once someone starts to invest, they then encourage them to invest more and more, often saying they cannot withdraw their winnings unless they invest a certain amount.
They then go silent - leaving many investors tens of thousands of pounds out of pocket.
How can I avoid being caught out?
The basic rule of thumb is that if it seems too good to be true, it probably is. So be wary of any investment offering high returns with supposedly little or no risk.
Alarm bells should also ring if you invest in any sort of scheme with which you are then pressured to invest more and more.
Hard-sell tactics, such as claiming that you will miss out if you do not invest immediately, are often a sign you are being duped.
I've been defrauded. What should I do?
Break off all contact with the fraudsters (or "brokers") behind the scheme, and report the scam to Action Fraud (0300 123 2040).
Then alert your bank to protect your accounts and, is possible, stop any payments yet to be made.
It is also worth holding on to any written communications you receive from a fraudulent trading website to share with the relevant authorities.
However, as most sites of this kind are unregulated, and many are based overseas, the chances of you receiving compensation from a UK scheme are slim.