Only a third of Britons would vote to stay in the European Union in a proposed referendum, a poll showed on Monday, Feb 18 underlining the scale of Prime Minister David Cameron's task in persuading voters to back his flagship EU policy, Reuters reports.

In one of the biggest gambles of his premiership, Cameron promised last month to win back powers from Brussels and hold a vote on Britain's 40-year EU membership. Cameron would campaign to stay in the EU, provided he can reform Britain's ties with Brussels in the face of opposition from European allies and he secures a second term in an election due in 2015.

Just 33 percent of voters would support his call to remain in the bloc, with 50 percent wanting to leave and 17 percent not planning to vote, according to the Harris Interactive poll in the Financial Times.

The last time Britain held a vote on staying in Europe was in a 1975 referendum when the "Yes" campaign came from behind to win by a wide margin, with 67.2 percent in favor of European membership and 32.8 percent against.