An anonymous investor has placed a bet on an index of Europe's top 50 stocks falling by a third by the end of September, as world equity markets plunged for a third day and volatility hit a three-year high.

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.