US STOCKS-S&P 500 futures fall as 'remain' barely leads in early UK vote count
* Sterling weakens against the dollar
* Opinion poll put "remain" campaign in lead
* Banks climb after Fed gives passing grade in stress tests
* Indexes up: Dow 1.3 pct, S&P 1.3 pct, Nasdaq 1.6 pct (Adds detail on market reaction, comment)
By Noel Randewich
June 23 (Reuters) - U.S. stock index futures dropped late on Thursday as early voting returns suggested Britain was on the verge of leaving the European Union, a move that investors fear could hurt the global economy.
S&P E-mini futures fell as much as 2 percent in after-hours trading before paring losses to trade down 1.2 percent.
Early results from the referendum showed margins were nail-bitingly tight, with the "remain" camp leading by less than 1 percentage point with about a quarter of voting areas counted.
Sterling fell as low as $1.4351 against the dollar, more than wiping out its earlier gains, which had lifted the pound above $1.50 for the first time this year on the back of a YouGov opinion poll that suggested Britons had voted 52-48 percent to stay in the EU. Continuación...