Prime Minister David Cameron's plan for Britons to vote on whether to leave the European Union is facing its first hurdle in Parliament, with support from the main opposition but unrest within Conservative ranks.

Lawmakers are debating the referendum bill, which lays out the rules for a vote by 2017 on the question, "Should the United Kingdom remain a member of the European Union?"

Kicking off the debate Tuesday, Foreign Secretary Philip Hammond said expansion of the now 28-member bloc since Britain voted to join in 1975 "has eroded the democratic mandate for our membership to the point where it is wafer-thin and needs to be renewed."

Cameron faces pressure from Conservative Euroskeptics, who want to be able to campaign for a "No" vote even if the prime minister disagrees.