Thousands of investors joined sharp institutions in making millions of pounds from the short-lived collapse in share prices that followed the terrorist strikes in London last week.
It was the busiest day of trading for over two years. The London Stock Exchange said 4.75 billion shares were traded on Thursday compared with the recent daily average of about 3.1 billion.
BP and Vodafone were among Britain's largest companies that took advantage of the volatile markets to improve their balance sheets.
Financial spread-betting firm City Index said more than 8000 retail investors had dived into the market on Thursday, correctly backing their hunch that share prices would quickly bounce back.
Some will find profiteering from horror distasteful. But many in the City applauded the resilience of capitalism.
The opportunity was created by automatic trading systems, used by institutional investors, that began dumping stock when shares fell sharply as news of the blasts spread through the City. At one point the UK benchmark stock index, the FTSE 100, had lost 100 points in two minutes and at its lowest was showing a 4 per cent fall.
"Despite the awful news, there were a considerable amount of people trading and making a lot of money," said City Index chief executive Clive Cooke. "We were deluged with buyers, it was absolute mayhem. In this electronic age, instead of being shell-shocked, investors got straight on to their PCs and took the opportunity to make money."