The Bank of England pledged a huge financial backstop to calm plunging markets after Britain voted to leave the EU and central banks around the globe intervened in markets, highlighting worries that volatility could quickly hit the world economy.

The BoE offered to provide more than 250 billion pounds ($347 billion) plus "substantial" foreign currency liquidity and it was ready to take additional measures if needed, Governor Mark Carney said on Friday after markets went into a tailspin.

The British pound fell by as much as 10 per cent to a 31-year low against the dollar in early trade and European shares were down close to 10 per cent before recouping some of their losses.

Central banks are concerned that market liquidity could quickly up dry from extreme swings, leaving the real economy without access to cash and financial instruments, eroding their growth prospects and dragging down growth.

"The Bank will not hesitate to take additional measures as required as those markets adjust and the UK economy moves forward," Carney said, warning that economic volatility can be expected as the UK adjusts.

Britain's economy was already slowing ahead of the referendum and Carney already warned that the economy could go into recession in the event of a vote to leave the EU.

In a rare move for a major central bank, the Swiss National Bank openly intervened in currency markets to weaken the safe-haven franc, promising to do even more if needed.

"Following the United Kingdom's vote to leave the European Union, the Swiss franc came under upward pressure," the SNB said in a statement. "The Swiss National Bank has intervened in the foreign exchange market to stabilise the situation and will remain active in that market."

Major Asian central banks were also said to be intervening, with traders suggesting that the Bank of Korea was seen to have sold dollars to curb the won's fall while the Reserve Bank of India likely sold dollars through state-owned banks to prevent the rupee from falling further.

The European Central Bank was expected to hold a governing council telephone conference call on Friday and make a statement later. Although the bank declined to comment immediately on Friday, ECB President Mario Draghi earlier said it was prepared for all eventualities.

Although European yields spiked - 10-year yields in Spain and Italy rose as much as 40 basis points in early trades - traders said that ECB's asset purchases, part of quantitative easing, had helped calm markets.

The biggest central banks in the world, including the Fed, the BoE, the ECB, the SNB and the Bank of Japan have standing swap facilities, an unlimited backstop to exchange currencies in case of market disruption.

First used after the 9/11 attacks in 2001, the swap lines were made permanent after the global financial crisis and can be activated by any of the banks. - Reuters