The House of Representatives has voted against the $700bn bail-out plan for the US economy - sending
The shock result came as Congress rejected the controversial, taxpayer-funded rescue package by a vote of 228 to 205.
The Dow Jones plunged by more than 7% on the news - just 3% less than an official crash. It closed down 777.68 at 10365.45.
Experts believe the FTSE 100 may well dive in response when London markets open tomorrow.
Today, banks including HBOS, Lloyds TSB and RBS were among the FTSE's biggest fallers. The index plunged 5.3% at 4818.8 - its lowest level since April 2005.
Prime Minister Gordon Brown said: "The vote in America is very disappointing. In recent times we in Britain have taken decisive action to maintain the stability of our system.
"The Governor of the Bank of England, the Chancellor and I will take whatever action necessary to ensure the continued stability for Britain."
As the events unfounded, Sky News presenter Jeremy Thompson said: "We are witnessing perhaps one of the most dramatic economic and financial moments in modern history - on both sides of the Atlantic."
He added: "Now the lawmakers are going away to assess where they go from here."
Democratic and Republican negotiators are continuing private talks in a bid to bring a similar bill for another vote before the House breaks.
The rescue package was seen by the US Treasury as the best way to rescue the US economy from financial meltdown.
But there had been widespread criticism that taxpayers' cash was being risked to prop up banks, who had caused the credit crunch in the first place with their irresponsible lending.
Earlier, President Bush admitted the bill would present a "difficult vote" for members of the US Congress, but urged them to pass it.
Tonight he said he was "very disappointed" that the plan was rejected.
He added: "We've got a big problem...Our strategy is to continue to address this economic situation head-on."
Sky's Robert Nisbet said: "It seems there was an overall fear from Congressmen and women, who were up for election come November, that they didn't want to be seen backing a deeply unpopular measure."
House Republicans blamed a partisan pre-vote speech by Speaker Nancy Pelosi, a Democrat, for the bill's failure.
She told a live news conference afterwards: "The legislation has failed, the crisis has not gone away. We must work in a non-partisan way in order to have another bite at the apple, in terms of some legislation."
Sky News