Stock index futures extended losses on Friday, suggesting the benchmark S&P 500 <.SPX> may open below its November 21 bear market low, as government data showed the economy contracted more sharply than initially estimated in the fourth quarter.

Investors fretted over which other banks might see similar action as Washington grapples to stabilize the banking sector and worries persist about dilution of current shareholders.

S&P 500 futures fell 21 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 tumbled 155 points, and Nasdaq 100 futures lost 24.25 points.

(Reporting by Ellis Mnyandu; Editing by James Dalgleish)