U.S. stock index futures rose on Sunday tracking gains in the British pound after weekend polls showed the campaign to keep Britain in the European Union had regained momentum ahead of Thursday's referendum.

Previous polls showing an increased probability that Britons would vote to leave the EU have rattled markets in recent days as investors worried about the implications to the global economy of a so-called Brexit.

Two opinion polls published on Saturday showed "in" regained the lead, while a third one showed a narrowing lead for the "out" camp.

The pound gained more than 1 percent against the U.S. dollar to trade above $1.45 for the first time in seven sessions. On Thursday it had touched a 10-week low of $1.401.

Campaigning for both sides of the British referendum resumed on Sunday after being suspended following last week's killing of Jo Cox, a lawmaker in favor of Britain remaining in the EU.

Specialist traders Specialist traders work on the floor of the New York Stock Exchange. Photo: REUTERS/BRENDAN MCDERMID

The overall picture regarding the vote continued to show a nearly even split electorate, and volatility in U.S. financial markets was expected to rise into Thursday's vote.

Futures snapshot at 19:45 EDT:

* S&P 500 e-minis were up 18.25 points, or 0.89 percent, with 32,130 contracts changing hands.

* Nasdaq 100 e-minis jumped 35.75 points, or 0.82 percent, in volume of 4,133 contracts.

* Dow e-minis gained 135 points, or 0.77 percent, with 4,779 contracts changing hands.