LONDON (Reuters) - Prime Minister David Cameron, campaigning for Britain to stay in the European Union, has the edge over London Mayor Boris Johnson, the most popular figure in the "Out" camp, in trying to sway the key group of undecided voters, a new poll indicated.
Evidence on which camp was ahead from another poll taken after Cameron's deal on new EU membership terms for Britain struck on Feb. 19 showed both sides were neck and neck.
The ruling Conservative Party is deeply split on the EU issue to be decided in a June 23 referendum, with Cameron and Johnson the figureheads of the opposing camps vying in particular for the support of moderate Conservatives, regarded as the pivotal, swing group of voters.
Overall, the latest ComRes telephone poll for the Daily Mail found that the "In" camp was ahead by 12 points at 51 percent, though its lead had narrowed since details emerged of Cameron's deal with the other 27 EU heads of government in Brussels.
An online YouGov poll for the Times found the sides were neck and neck, reflecting a trend that has been apparent for several months whereby phone polls have found "In" far ahead while online polls have found much closer results.
The YouGov poll, which had the "Out" camp one point ahead at 38 percent, suggested a drop in support for the "Brexit" option, which had been ahead by nine points in a poll published on Feb. 5, before Cameron's deal was finalised.
ComRes interviewed 1,000 people between Friday and Monday, while the YouGov poll of 3,482 people was conducted between Sunday and Tuesday. Both straddled key news developments.
Cameron clinched his EU agreement late on Friday night and announced on Saturday that the official government position was to campaign for an "In" vote. Britain has been in the EU since 1973 and has its second largest economy.
Six members of his cabinet defected to the "Out" side on Friday and Saturday, while on Sunday Johnson, who is not in the cabinet but has far greater popular appeal than those who are, came out for Brexit in a blaze of publicity.
ComRes analyst Adam Ludlow said Johnson was important because of his appeal to moderate Conservatives. Half (52 percent) of Conservative voters say they may change their mind, more than enough to change the June 23 outcome, ComRes says.
Its poll found two thirds of Conservative voters (68 percent) had a favourable opinion of Johnson, while 88 percent viewed Cameron favourably.
"Johnson can carry a message and moderate Conservative voters will consider it rather than dismiss it out of hand," Ludlow wrote. "His affableness may also soften the perceived threat of leaving the EU for some less committed 'remain' supporters."
"There is of course one towering figure respected more than any other by moderate Conservatives, whose endorsement for remaining in the EU is more important than any other: David Cameron."
The divergence between the online and phone polls, coming after all pollsters got last year's general election wrong, has raised uncertainties in a context where Brexit fears have knocked the sterling currency to a seven-year low.
JP Morgan said in a research note that it was inclined to put a little more weight on the soundings done by phone.
"They may be less susceptible to biases arising from attracting highly politically engaged voters, and they also tend to have around half as many undecided people -- potentially offering a little more clarity about which side of the fence the swing voters are coming down on."
But YouGov's Anthony Wells and Stephan Shakespeare argued that in person-to-person phone interviews people felt pressured to give an opinion even if they didn't hold it that firmly, and were more likely to say they favoured the status quo.
They maintained that the phone polls may be over-stating the "remain" vote as a result.
The British Social Attitudes Survey, a face-to-face survey using a random sampling method believed to give the most accurate results, found in its most recent research that 60 percent wanted to stay in the EU while 30 percent wanted to leave.
Writing by Estelle Shirbon; Editing by Guy Faulconbridge and Mark Heinrich