William Hill cut its odds of Britons voting to remain in the European Union in a June 23 referendum to 1/5, the lowest level to date and indicating an implied probability of 83 percent, a spokesman said.

"William Hill have slashed their odds for a 'Remain' vote in the EU Referendum to their shortest level," a spokesman said.

"We are now offering 1/5 Remain," the spokesman added.

As polls have given sharply different pictures of public opinion, many investors are watching betting odds closely. Odds have consistently indicated a high probability of an "In" vote in the referendum.