The bookmaker is now offering odds of 7/4, or a 36 per cent probability, for a bet on an Out vote. It was offering odds of 4/9 for a bet on an In vote, or a probability of 69 percent.
The group has also lengthened the odds for a Remain vote from a shortest bet of 1/7 (87.5 per cent chance) to 4/9 (69 per cent chance) - and said the odds have "never been longer since the referendum date was announced".
William Hill has also cut the odds for England to vote Leave while Scotland votes Remain from 2/1 to 13/8.
"Political punters betting with William Hill that the EU referendum will produce a 'Leave/Brexit' outcome have forced the odds down to their shortest level since the date of the vote was confirmed back in February," William Hill spokesman Graham Sharpe said.
Bookmakers have been adjusting the odds on the results of the upcoming referendum a polls showing differing likely outcomes continue to emerge.
And at the end of last month, William Hill slashed the odds on Prime Minister David Cameron leaving Number 10 before the end of this year.
Meanwhile, Betfair betting odds now place a 64.5 per cent implied probability on a pro-EU vote winning the referendum, a near 14-point fall since last Thursday, the FT reported today.