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The poll was carried out as Britain's ruling Conservative party started to descend into civil war following the resignation of former Tory leader Iain Duncan Smith (IDS) from his role as welfare secretary.
IDS is prominent eurosceptic and after his departure, bookmakers had already shortened the odds of a Brexit happening. The theory is that the more vocal Tory eurosceptics become against their pro-EU leader David Cameron, the worse it will be for the Remain campaign.
The poll was also carried out before yesterday's terror attacks in Brussels which killed at least 30 people. Following the attacks, bookmakers narrowed their odds again for a Brexit - which means they believe terror attacks in other EU countries are likely to push people towards voting for Leave.