* 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.
"These markets are thin, liquidity is poor and a recovery is obviously possible, but those traders who were looking to book a quick profit before a restful night's sleep have had their ideas shattered," said Jeremy Cook, chief economist at the international payments company World First.
Earlier on Thursday, U.S. stocks rose as investors bet the United Kingdom would remain part of the European Union, potentially avoiding damage to European trade and its consequences for the global economy.
In extended trade, Goldman Sachs Group added 1.17 percent and Bank of America climbed 1.5 percent after the Federal Reserve said all 33 stress-tested U.S. banks met minimal capital requirements.
In regular trade earlier, the Dow Jones Industrial Average rallied 1.29 percent to end at 18,011.07 points, its highest level since late April. The S&P 500 surged 1.34 percent to 2,113.32, just 0.8 percent short of its record closing high hit in May last year. The Nasdaq Composite added 1.59 percent to 4,910.04.
The financial sector led the S&P 500 stock index with a 2.1 percent gain.
With investors optimistic about a victory for Britain's "Remain" campaign, the CBOE Volatility index fell 18.5 percent, its largest daily percentage decline since October 2013.
Software maker Twilio Inc nearly doubled in its market debut, rising as high as $29.60 after pricing at $15. It closed up 91.9 percent at $28.79.
About 6.4 billion shares changed hands in U.S. exchanges, below the 6.8 billion average over the past 20 sessions.
Advancing issues outnumbered declining ones on the NYSE by a ratio of 4.98-to-1, and on the Nasdaq, a 3.57-to-1 ratio favored advancers.
The S&P 500 posted 52 new 52-week highs and two new lows; the Nasdaq recorded 88 new highs and 26 new lows. (Additional reporting by Richard Leong and Rodrigo Campos; Editing by Cynthia Osterman)