The implied probability of an In vote fell on Tuesday to around 55 per cent but then recovered to around 62 per cent later in the day, the bookmaker said.
The slight improvement in the forecast for the Remain campaign comes as some of the world's biggest banks make plans to draft in senior traders to work through the night on the 23 June, in what is expected to be one of the most volatile 24-hour periods for global markets in the last 25 years.
Citi, Deutsche Bank, JPMorgan, Goldman Sachs, HSBC, Barclays, Royal Bank of Scotland and Lloyds are among those banks planning to have senior staff and traders working or on call in London as results come in on the night, according to Reuters.
Both Citi and JP Morgan have warned in recent weeks that they will potentially be forced to cut jobs in the UK if the country votes to leave the EU. And JP Morgan boss Jamie Dimon has been vocal in his support of a Remain vote, last month urging shareholders in the bank to vote to stay in the union as it would be better for the British economy.
The Bank of England will also be staffed overnight, with senior policymakers on call if markets go into meltdown.