Stock futures slump after Britons vote to leave EU

S&P 500 futures and Nasdaq futures were down about 3.9 percent while those on the Dow Jones industrial average were off 3.1 percent, indicating Wall Street will open sharply lower. Amid the turmoil, sterling hit a 31-year low in its biggest intraday percentage fall on record and Prime Minister David Cameron said he would step down by October. Trading in S&P 500 and Nasdaq futures was halted briefly overnight after they fell more than 5 percent, triggering limit thresholds.