Fresh fears of a global recession have pushed London's leading shares index down more than 1%, capping off its worst quarter of trading for nine years.
The FTSE 100 Index fell 68.4 points to 5128.5 after worrying Chinese and US economic data added to fears about a Greek default.
The fall means London's blue chip shares have fallen 13.7% in the third quarter of 2011 - its worst performance since 2002 when the dot-com boom ended.
The UK's biggest companies have had some £212 billion wiped off their value in the past three months.
World markets have been hugely volatile in recent weeks as investors panicked that the US and eurozone would be unable to keep up with payments on their huge debts and would lead the world back into recession.
The latest falls were caused after a monthly survey by banking giant HSBC showed that China's manufacturing remained stagnant in September due to sluggish demand both at home and abroad.
Poor data from the US added to the gloom today after the Commerce Department said incomes fell for the first time in nearly two years in August.
This added to fears about the eurozone debt crisis, as Greek Prime Minister George Papandreou pressed European leaders, including French President Nicolas Sarkozy, to release the next eight billion euro (£7 billion) bailout instalment for his country.
Greece has warned it will default on its debt payments if it does not receive the money, which would create financial chaos.