U.S. stock index futures fell to session lows on Thursday after beleaguered U.S. automaker General Motors Corp said there was substantial doubt about its ability to continue as a going concern.

* GM also said if it was unable to successfully reorganize and debtor-in-possession financing was unavailable, it would be forced to liquidate under Chapter 7 of the U.S. bankruptcy code.

* S&P 500 futures fell 13.10 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 90 points, and Nasdaq 100 futures lost 9 points.

(Reporting by Ellis Mnyandu; Editing by Theodore d'Afflisio)