LONDON (Reuters) - The campaign for Britain to leave the European Union maintained its lead ahead of the June 23 referendum, although the gap narrowed to 1 percentage point, according to a monthly opinion poll by BMG Research published on Wednesday.
The poll showed 45 percent supported Britain leaving the bloc, unchanged from a similar poll last month, while backing for an "In" vote rose 1 point to 44 percent.
The online survey of 1,638 voters, carried out between May 20 and 25, found 12 percent of respondents were undecided.
The poll continued a trend of telephone surveys giving the "In" campaign a healthy lead, while those conducted online put the two camps neck and neck.
Britain's leading expert on elections said earlier on Wednesday he was "none the wiser" about which polls were providing a clearer picture of Britons' voting intentions.
But commentators agreed that turnout would be a crucial factor in determining the final outcome because of the divide between generations, with young voters, who have a poor voting record, strongly backing staying in the EU, but older, more reliable voters favoring an exit.
The BMG poll found that six out of 10 of those in the 18-24 age group backed staying in the EU, but just 47 percent said they would definitely vote.
But 80 percent of those aged over 65 said they would definitely vote and six out of 10 in that age bracket indicated they would vote for a Brexit.
(Reporting by Michael Holden in London and Parikshit Mishra in Bengaluru; Editing by Catherine Evans and Peter Cooney)