Sterling sank to a seven-year low today as companies and investors rushed to insure themselves against the chances of a British exit from the European Union that HSBC said could knock 15 percent off the value of pound.

The aftermath of David Cameron's announcement of a June 23 referendum on "Brexit" has driven the worst three days for the world's fourth most traded currency since the depths of the financial crisis in 2009.

Another wave of selling in London this morning drove it to less than $1.39, within 4 cents of levels not seen since it sank to parity to the dollar in the mid-1980s.

"There are very few people willing to take the other side of the move lower," said Josh O'Byrne, a strategist at Citi. "Knowing now that the vote will be in June, there is a greater incentive for those that haven't hedged GBP exposure to do so."

The cost of currency derivatives, which allow companies, big institutional investors and hedge funds to protect future revenues or asset values held in sterling against sharp swings in the currency, jumped to their highest in more than four years.

HSBC, Britain's biggest bank, said the currency could lose up to 15 percent of its value and UK economic growth could be up to 1.5 percentage points lower next year if Britain votes to leave.

The latest poll showed the "In" camp only narrowly ahead at 51 percent, versus 39 percent for "Out", with 10 percent undecided. Other polls have been even closer.

Sterling fell to $1.3878 with analysts saying the 2009 low of $1.35 now a real prospect. It fell 0.6 percent to its weakest in 16 months against the euro.

The Bank of England's broader trade-weighted measure of the currency's broader strength fell 1 percent to its lowest in two years.

"A vote for Brexit would have potentially huge consequences for all asset classes," HSBC said.

"Following a vote to leave we think uncertainty could grip the UK economy, triggering a potential slowdown in growth and a collapse in sterling."

So far, however, the reaction on other UK markets has been far more muted.

Government bond prices are steady on the week, any nervousness among investors holding UK assets offset by the prospect that the vote will further push back any rises in interest rates by the Bank of England.

Governor Mark Carney said yesterday that the bank could still use rate cuts and a broadening of a bond-buying programme to boost Britain's economy if needed after the Brexit vote.

Gilt yields have edged lower for four out of the last five trading sessions and have moved mostly in tandem with German Bund yields. The 10-year gilt yield is so far down around 5 basis points this week, at 1.364 percent. London's stock exchange has also seen a far more mixed response. Some property companies have been targeted as potential losers from the vote, while others have benefited from the assumption that a weaker pound would help exports.