US STOCKS-S&P 500 futures down as 'Leave' leads in early UK referendum count
* Sterling falls against the dollar
* Opinion poll puts "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 fell in after-hours trading on Thursday as early results from Britain's bitterly contested referendum showed a lead for supporters of leaving the European Union.
S&P E-mini futures fell 0.8 percent as early results showed the "leave" campaign with 52.6 percent of the vote and the "stay" camp at 47.4 percent. But it was too early to establish a firm trend in a contest that remained too close to call.
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.
Reflecting nervousness among investors, futures on the VIX volatility index rose 1.02 points to 17.7. Continuación...