Stock index futures fell on Thursday as doubts about a quick economic rebound fueled a global equity sell-off, while Britain's reduced rating outlook pointed to more fallout from the credit crisis.

Asian markets dipping overnight after the U.S. Federal Reserve gave a more pessimistic prognosis for economic recovery. In Europe, U.K. stocks were a top casualty as the FTSE 100 index <.FTSE> dropped more than 2 percent after Standard & Poor's cut its outlook on Britain to negative from stable.

The outlook downgrade of U.K. debt is being taken pretty negatively, said Rick Meckler, president of investment firm LibertyView Capital Management in New York. It sets a precedent for what could start to happen to a lot of the world, given the amount of spending that's going on.

Investors were also cautious ahead of a government report on weekly jobless claims that could shed more light on the state of the labor market. The data is due at 8:30 a.m. EDT.

S&P 500 futures fell 6.30 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures shed 43 points, and Nasdaq 100 futures dipped 9.75 points.

Bank of America Corp is among stocks in the spotlight after the Financial Times reported the largest U.S. bank wanted to pay back $45 billion in government bailout funds by the end of the year, accelerated by a program to raise capital.

Former Federal Reserve Chairman Alan Greenspan said the U.S. economy and financial markets had improved, but warned that banks faced a capital shortfall, which could stall lending and obstruct a recovery, Bloomberg reported Thursday.

(Reporting by Ellis Mnyandu; editing by Jeffrey Benkoe)