The FTSE 100 index of leading shares closed with heavy losses of more than 5.0 percent Wednesday as investors brushed aside central bank efforts to counter the global financial crisis.
London shares shed 5.18 percent in turbulent trade to finish at 4,366.69 points.
The Royal Bank of Scotland was the most traded stock, seeing 441 million units change hands, followed by Vodafone which saw 318 million shares switch owners.
The falls came despite an unexpected half-point interest rate cut by the Bank of England to 4.5% -- part of an unprecedented synchronised effort by central banks around the world.
The cut is Britain's biggest rate reduction for seven years and it comes amid the growing threat of a recession.
The shares rally that followed the rate cut announcement proved to be shortlived, however.
Earlier in the day, the government announced it would pump 50 billion pounds into the country's main banks as part of an emergency bailout package worth hundreds of billions of dollars.
The government's three-part package also makes available 200 billion pounds in short-term loans and another 250 billion pounds to guarantee loans between banks.
It is hoped the measures will overcome the reluctance of banks to lend to each other, which is at the root of the current crisis.
Eight banks -- HSBC, Barclays, Royal Bank of Scotland (RBS), Lloyds TSB, Standard Chartered, HBOS, Abbey and Nationwide Building Society -- are covered by the package.
The day's biggest loser was retailer Sainsbury, down 47 pence -- or 14.9 percent -- to close at 267.75.
It was followed by miner Vedanta Resources, which lost 141 pence -- or 14 percent -- to close at 864.
On the upside, bank HBOS clawed back some recent losses and topped the leaders board on Wednesday. It gained 23 pence -- or 24.5 percent -- to close at 117.
The day's second-best performer was TUI Travel, which added 8.25 pence -- or 4.05 percent -- to close at 212.
Sterling was down against its major rivals by 16:58 BST. The pound slipped to 1.2652 euro (from 1.2855 euro by the close on Tuesday).
Sterling stood at 1.7307 US dollar by late afternoon, compared to 1.7509 at yesterday's close.