European Union
File picture shows European Union member states' flags flying in front of the building of the European Parliament in Strasbourg, April 21, 2004.Reuters

Britons are more likely to vote to stay part of the European Union than leave in a future membership referendum, a poll showed on Monday, with only 18 percent saying they would definitely elect to exit the bloc.

The YouGov poll for The Times newspaper asked the question because Britain faces a close national election on Thursday and Prime Minister David Cameron has promised to hold such an in-out referendum if re-elected.

Such a referendum has not been held in Britain since 1975, when the country first joined what was then the European Economic Community. The bloc was approved by a margin of 2-1.

Monday's poll, which surveyed 1,823 adults from April 28-29, found that 34 percent of Britons would definitely vote to stay in the EU, and that a further 18 percent would probably vote to stay in but were also open to persuasion.

By contrast, just 18 percent said they would definitely elect to leave the bloc, with a further 14 percent saying they would probably vote to exit but were also open to persuasion.

Sixteen percent were undecided.