The pound fell sharply after opinion polls suggested support for the UK leaving the EU is growing.
Sterling hit a three-week low against the dollar, dropping more than 1.5 cents to $1.4358.
Against the euro, the pound was nearly one euro cent lower at €1.2677.
A YouGov poll found 45% favoured the UK leaving the EU, with 41% wanting to stay. An Observer/Opinium poll also found the Leave campaign ahead by 43% to 40%.
“The polls are likely to make people rather uneasy and we can see that quite clearly today in the pound,” said Craig Erlam, senior market analyst at Oanda.
“With both sides likely to step up their game over the next couple of weeks, I imagine we’ll see a lot more volatility in the pound and the closer the polls get, or if ‘Vote Leave’ continues to push ahead, the pound may find itself back towards April’s lows before too long.”
On the stock market, the FTSE 100 index opened up 49.83 points, or 0.8%, at 6,259.46.
The index was lifted by mining shares after prices of metals rose. The price of copper hit a four-week high, and among the major mining companies, Rio Tinto, BHP Billiton and Anglo American were all up by 3%-4%.
The price of copper touched $4,730 a tonne at one point overnight, the highest level since 12 May, after the dollar fell following the release of surprisingly weak US jobs figures on Friday.
