Risk reversals are a gauge of investor expectations for a currency's direction and are used to hedge against expected moves. The fall in sterling risk reversals indicates greater demand for put options, derivatives that give investors the right to sell an asset.

Call options offer the right to buy.

The fall in nine-month and one-year sterling risk reversals suggests investors are rushing to protect themselves against more sterling downside between now and into 2019 by buying put options.

The options move came after the pound plummeted on spot markets to its lowest level in nearly a year on investor angst that Britain is headed for a cliff-edge Brexit. Britain is due to leave the European Union at the end of March 2019.