Support for Britain to stay in the European Union rose in the last month, suggesting a crisis in the steel industry and attacks in Brussels have not boosted the campaign for an exit, according to an ORB poll for the Daily Telegraph.

The survey found 51 percent of Britons backed remaining in the 28-member EU with 44 percent in favour of leaving, with 5 percent undecided. The country will decide on EU membership in a referendum on June 23.

Support for the "In" campaign was up by four percentage points compared to a similar poll carried out last month, which the Telegraph said indicated that the warnings of Prime Minister David Cameron and others who want Britain to stay in the EU that an exit would impact the country's economy.

"Those campaigning to leave Europe are inviting the British people to make an extraordinary choice - to be the first major economy in history to deliberately choose a second-rate, more restrictive trading relationship for its biggest market," Cameron wrote in an article for the paper.

Last month's deadly militant attacks in Brussels in which 32 people were killed and the decision of Tata Steel to put its loss-making Welsh plant up for sale had been predicted to increase support for an EU exit.

The ORB poll of 800 people did show the two campaigns were running neck and neck among those who said they would definitely vote in June, although even among these voters the "In" campaign was up four points on 49 percent compared to last month with "Out" which was down four points to 48 percent.

Lynton Crosby, Cameron's election strategist who helped his Conservatives to victory in last year's election, said the referendum would hinge on which side could secure a better turnout among its own supporters rather than convincing those who had not yet decided how to vote.

"Because of the small number of voters who say they could be persuaded to change their minds, the campaigns must redouble their efforts in effectively communicating with their supporters," Crosby wrote in the Telegraph.

"This could lead to the debate becoming more polarised over the coming weeks."

The Telegraph said the poll was taken in April.