LONDON (AP) — The pound is hovering at seven year lows against the dollar as investors try to hedge bets ahead of the upcoming vote on whether Britain should remain in the European Union.

The currency fell to $1.3987, the lowest level since March 2009. The pound first began to drop after London Mayor Boris Johnson announced over the weekend that he would join the campaign to leave the EU.

The pressure comes as former service chiefs argued in a letter in the Telegraph newspaper on Wednesday that Britain was in a stronger position to deal with security threats from within the EU.

The former Royal Navy, Army and Air Force leaders says they were worried about the rise of the Islamic State group, instability in the Middle East and Russian nationalism.