U.S. stock futures fell sharply in premarket trading on Friday in the wake of Britain’s vote to quit the European Union, catapulting global markets into turmoil.

S&P 500 futures and NASDAQ futures were both down about 3.6 per cent, while those on the Dow Jones industrial average were off 2.8 per cent.

The drops suggested Wall Street will open with steep losses, with the historic referendum delivering the biggest shock to the global financial system since the 2008 financial crisis.

As markets reeled, British Prime Minister David Cameron said he would step down by October.

Investors worried about damage to the world economy sought refuge in the dollar and other safe-harbour assets such as gold and U.S. Treasury bonds.

British markets plummeted, with sterling hitting a 31-year low in its biggest intraday percentage fall on record. Britain’s FTSE 100 stock index slumped 6.5 per cent in early trade. Asian equity markets also tumbled.