The mystery investor has bought put option contracts on the DJ Eurostoxx 50 index that will result in a profit if it plunges to 2,800 or below by the end of September. Based on the 2,800 strike price, the position covers a notional €6.9bn, and potentially even more using a market price of about 4,100 when the trades were done on Tuesday and Wednesday.

The identity of the investor is unknown but market sources speculated it was either a large hedge fund hedging itself against deepening losses, or a long-only fund manager pressing the panic button to protect its gains.

The investor has bought a total of 245,000 put options on the index. The September put option with a 2,800 strike was the most popular DJ Eurostoxx 50 contract yesterday, according to data from Bloomberg.

Volatility in European equity markets has risen sharply this week as investors cut back on the amount of risk they are taking. The VSTOXX index, which measures the volatility of the DJ Eurostoxx 50 index, hit 34 this morning, which is more than double its three-year average.

Similarly the volatility of the US stock market was trading at almost three times its three-year average, hitting 30 yesterday.

However, both indices continue to trade below their 2002 highs.

European stock markets were trading down almost 3% at by 13:00 GMT today, after large drops in Asia and Australia overnight. The Australian market fell 300 points at one stage when futures trading was suspended for over an hour and traders were forced to hedge positions by selling physical stocks rather than futures.

An analyst at Goldman Sachs JB Were in Australia wrote: "I think I shall remember this day as the day that I saw the market go to hell, look into the abyss - didn't like what it looked like and then came screaming back up as far away from there as it could get. ... It was a truly spooky day and I’ve seen a lot over the last 20 years but today will be one that anyone who saw it will never forget. But this is what market bottoms are made out of."

The rise in volatility and risk aversion has also contributed to a sharp appreciation in the Japanese yen, which has been used to finance the so-called carry trade, where investors borrow in a low-yielding currency to invest in one with a higher-yield.

Analysts' belief that the yen carry trade is set for a major unwinding has intensified today as the Japanese currency continued to rally in morning trade.

The yen strengthened today as it broke through several psychological barriers. The yen hit 113.60 against the dollar by 12:35 GMT, the first time in more than a year it has dropped below 114. The yen was substantially up against the dollar from yesterday, when it traded at above 116.

Simon Derrick, head of currency research at Bank of New York Mellon, said: "With any hope of even a brief bounce emerging in the yen crosses evaporating in the fierce glare of another horrible close in New York, it is clear that the vicious, self-reinforcing, downward spiral we were worrying about is already firmly established."