The FTSE 100 Index traded below 5000 today amid nerves that the recent surge in stock markets had run ahead of a fragile economic recovery.
London's blue-chip shares, which approached year-highs on Monday, dipped below the psychological benchmark for the first time since September 15 as downbeat data from the US turned trading screens red.
US jobless claims, and manufacturing figures, were far worse than expected - signalling the world's biggest economy was still a long way from a strong recovery.
This set back Wall Street's Dow Jones Industrial Average 2% overnight and triggered falls in Asian markets.
The Footsie's fall comes just two days after it registered its best-ever quarter - rising almost 21% between July and September.
Now investors are nervously eyeing key jobs data for September from across the Atlantic due this afternoon. These could trigger further falls if they come up short of market hopes.
IG Index's chief market strategist David Jones said: "The fall below 5000 is not going to do much for sentiment but it is going to add to the fragile sense we have got at the moment.
"The problem is we have risen 20% since July and there has always been that nagging doubt that we have got ahead of the fundamentals."
The biggest faller in London was insurer Legal & General, down more than 5% after strong gains in recent days on takeover speculation.
But it was another poor session for fragile banking stocks and miners which did most of the damage.
Part-nationalised Royal Bank of Scotland and Lloyds Banking Group were down 4%, but several miners were hit by a stronger dollar putting pressure on metal prices and worries that a weak recovery would hit demand for commodities.