A new opinion poll in the UK has dealt a heavy blow to Prime Minister David Cameron by showing that a majority of Britons favor leaving the European Union amid concerns over immigration.

The survey done by polling firm Survation for the Mail on Sunday found that if a referendum was held tomorrow 51% of Britons would vote to quit the EU against 49% who would vote to remain.

The findings differ from a July poll which gave the "in" camp a 54% advantage to 46% of the “out” camp.

The survey is likely to set alarm bells ringing in Downing Street ahead of David Cameron's promised referendum, which is due to take place before 2017.

Survation said the margin in favor of leaving the EU could grow if the migrant crisis gripping Europe continues, with 22% of those who said they would vote to stay indicating they could change their minds if the situation worsened, Skynews said.

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

Now Javier Farje, editor and commentator, says a possible UK exit from the EU will affect both sides significantly and have ‘disastrous’ consequences for the British, as it will impact their economy, work flow and other sectors of the society.

Back in July, British media reports said the UK would hold a referendum on the country's future membership in the European Union in June 2016.

The UK premier has already said he would campaign in favor of remaining in the EU, but has been pressing for restrictions on EU migrant access to welfare payments in the UK, increased powers for London, and the ability to avoid closer political integration.

EU membership has long been a contentious topic in Britain. Euroskeptics, who believe the UK would be better off outside the political and economic union, seek the UK withdrawal from the EU.