LONDON: A majority of Britons now favour leaving the European Union amid concerns over immigration, an opinion poll showed on Saturday, signalling a shift in views ahead of a referendum on Britain's membership of the bloc.

The survey, by polling firm Survation for the Mail on Sunday newspaper, found 51 percent of respondents wanted to leave the EU and 49 percent wanted to remain, excluding undecided voters.

While the results are within the poll's margin of error and represent a statistical tie, the previous comparable poll, carried out in late June and early July, had found support for staying in the EU at 54 percent while 45 percent wanted the country out of the 28-nation bloc.

Prime Minister David Cameron has promised to renegotiate Britain's EU ties ahead of the referendum which is due to take place before 2017.

But the anti-EU UK Independence Party says the government cannot address the freedom of workers from within the EU to come to Britain under one of the bloc's core principles.

Survation said the Mail on Sunday poll was the first time it had found a lead for the "out" campaign since November 2014.

The polling firm also said a "significant minority" of voters who favour remaining in the EU would consider changing their minds should Europe's migration crisis worsen.

The latest online poll was conducted on Sept. 3 and 4 and heard the opinions of 1,004 adults, a smaller sample size than the previous poll.