More people say they support Britain staying in the European Union than at any point since 2010, according to YouGov PLC's tracker poll.
The survey found 45 percent of respondents saying they would vote in a referendum to stay in the EU, against 35 percent who said they'd back leaving.
The shift may reflect the increased association between a possible British exit from the bloc and the anti-immigration U.K. Independence Party, the strongest opponent of membership, according to YouGov pollster Anthony Wells.
"As UKIP have got a higher profile, and the case for leaving becomes ... more closely associated with them, it could be that people don't like saying they want to leave because they don't want to sound like a Ukipper," Wells said in an interview. "They're damaging the very cause they're supposed to champion."
Conservative Prime Minister David Cameron has pledged to renegotiate Britain's terms of membership and then hold a referendum on leaving if he's still in office after the May 7 general election. UKIP is attracting the support of about 15 percent of the electorate, according to opinion polls.
YouGov questioned 1,772 adults on Sunday and Monday for the EU poll. It didn't specify a margin of error.