Detail of a European map, including Great Britain, is seen on the face of a Euro coin in London, Britain, January 31, 2016. REUTERS/Toby Melville

LONDON (Reuters) - The campaign for Britain to leave the European Union opened up its biggest lead in more than a year over the rival ‘in’ campaign, according to a poll conducted by YouGov to gauge public opinion ahead of a referendum.

The survey of 1,735 people showed 42 percent of Britons would vote to leave the bloc compared to 38 percent who wanted to stay - a 4 percentage point gap that is the largest in favour of a British exit seen in a YouGov poll since October 2014.

The gap widened as support for remaining within the bloc dipped to 38 percent from 41 percent in December. The proportion backing an exit was unchanged since its last poll while those who were undecided rose 3 percentage points to 18 percent.

Prime Minister David Cameron has promised to hold a referendum on Britain’s EU membership by the end of 2017 after attempting to negotiate reforms to address voter concerns about issues such as sovereignty and immigration.

Those renegotiations reached a critical point on Sunday as Cameron demanded more from his EU partners ahead of a meeting with the bloc’s lead negotiator Donald Tusk. Cameron hopes to seal a reform deal in February and could hold the referendum as early as June.

Surveys conducted by a range of pollsters show public opinion is split on the issue. A separate poll on Saturday showed that most Britons backed staying in the bloc, but support for an exit was rising.