* Britain's FTSE drops 4.5 percent
* Banks hit by growth fears, funding worries
* Commodities fall on slowdown concerns
By Joanne Frearson
LONDON, Aug 18 (Reuters) - Britain's top shares dropped on Thursday in their worst one-day fall since March 2009, led by banks on concern the growth outlook for global economies was slowing and worries about contagion in the euro zone debt crisis.
Stocks extended falls after August factory activity in the U.S. Mid-Atlantic region dropped to its lowest level since March 2009, adding to earlier worries the U.S. economy was slowing down following higher than expected weekly jobless claims.
Banks tumbled to become the worst performers, with the down 6.9 percent - also its biggest daily drop since late March 2009.
Barclays , Royal Bank of Scotland and Lloyds Banking Group dropped 9.3 to 11.5 percent.
The sector was also rattled by a Wall Street Journal report that the U.S. Federal Reserve was examining in closer detail the U.S. units of big European banks on worries about their short-term funding due to contagion fears from the euro zone debt crisis.
"The banks are plugged into the euro zone sovereign debt crisis," said Mike Lenhoff, chief strategist at Brewin Dolphin. "But it is not just worries about the euro zone, it is growth in the United States as well."
"The concern is that global economies will go back into recession, and downgrades to economic growth are likely to translate into cuts to earnings."
The UK benchmark index closed down 239.37 points, or 4.5 percent at 5,092.23, knocking about 60 billion pounds ($99 billion) off the index, with volume 132 percent of its 90-day daily average.
The index has slumped 16.3 percent since the July to August sell-off began and made its biggest one day drop since March 2, 2009.
It also dropped below both the 38.2 percent retracement or 5,284.96 and 23.6 percent retracement or 5,096.17 of its July to August sell-off, with the next support level seen at the psychological 5,000 mark.
DOWNGRADE TO GROWTH
Investors had early on Thursday been concerned about a slowdown in the United States and Europe after Morgan Stanley cut its global growth forecast for 2011 and 2012 citing both regions as "dangerously close to a recession."
"The anaemic post-recession recovery and the pronounced market volatility indicate that both the US and parts of Europe are on an unsustainable path", said Jack Malvey, chief global market strategist for BNY Mellon Asset Management.
"The worst case would be a mild, brief recession, but we are more likely to experience a low-growth recession over the next three to six months."
Also compounding the worries early on that the global recovery was losing steam was news that Deutsche Bank had cut its expectations for Chinese GDP growth due to the slowing economic environment in the United States and Europe.
Commodity stocks whose performance is strongly correlated to global growth were also among the worst performers, tracking crude and base metal prices lower.
Of the mining stocks Xstrata was the hardest hit, losing 10.2 percent, while Cairn Energy fell the most among the oils, down 8.7 percent. (Reporting by Joanne Frearson; Editing by Hans-Juergen Peters)
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