William Hill, the UK's largest bookmaker, is now offering odds of 2/9 on the UK voting to stay in Europe, equivalent to an 81 per cent chance of Remain winning.
In contrast, the odds of a Leave vote are now 3/1, giving a 25 per cent chance that the country will vote for Brexit.
Ladbrokes is offering slightly lower odds of 1/4, at 76 per cent, that Britain will opt to stay, up from 73 per cent on Monday, while it has lowered the Leave odds to 3/1 (24 per cent).
While the bookmakers' stats would indicate the UK is strongly in favour of Remain, both sets of figures contradict a Financial Times poll which was also released today.
The FT poll showed Leave had claimed a one per cent advantage, with Brexit having claimed 45 per cent of the vote to Remain's 44 per cent (although this left 11 per cent undecided).
"On the eve of the vote we see a very similar scenario to last year’s General Election, with polling showing a dead heat but the betting demonstrating a significant tilt towards one outcome: a vote to Remain in the EU," Matthew Shaddick, head of political betting at Ladbrokes, said.
Ladbrokes estimates £100m will have been wagered by the time the polling booths close on Thursday night in the biggest non-sporting bookmaking event in history.
"Three quarters of all stakes at Ladbrokes yesterday were for that outcome and we see a 76 per cent chance of that result materialising."
Meanwhile, William Hill spokesman Graham Sharpe said: "66 per cent of all the money gambled with William Hill on the outcome has been for Remain with bets of up to £100,000 staked, but 69 per cent of all individual bets placed have been for Leave.
"There has been worldwide betting interest – we have taken bets from Andorra, Argentina, Austria,Canada, Finland, Holland, Japan, Malaysia, Malta, Russia, Sweden, Switzerland, Ukraine, to name just a few countries."