Those in favour of Brexit are more likely to vote in the coming referendum on Britain's membership in the EU.
As such, the Leave campaign may have a decisive advantage when the UK comes to vote on 23 June.
A poll by the Daily Telegraph found that, without taking into account likelihood to vote, the In and Out campaigns are virtually tied - 47 per cent said they would vote In and 49 per cent would vote Leave.
However, when likelihood to vote was taken into account, 52 per cent said they would vote to leave the EU, while 45 per cent said they would vote to stay.
In total, 79 per cent of those intending to vote Out said they were certain to vote, with 72 per cent of In voters saying the same.
The poll also found the "potential for uncontrolled or increased immigration" is the chief concern of a third (31 per cent) of undecided voters.
A recent survey by the Confederation of British Industry (CBI) found four in five CBI members believe the UK should remain in the EU.
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