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

Lawmakers held their first debate on 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?"

Introducing the legislation, 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."

"We need a fundamental change in the way the European Union operates," he said, with more protections for Britain and other countries whose economies are not bound together by the euro single currency.

Cameron says he will argue for Britain to remain in the EU if he succeeds in renegotiating the terms of membership. He wants to cut back the political powers the EU has over member states on sensitive issues including welfare and immigration.

The bill calls for a referendum by the end of 2017, but Hammond said it could be held sooner if negotiations go quickly.

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

The referendum bill is likely to pass second reading Tuesday, a key stage in its progress into law.

The Labour and Liberal Democrat opposition parties say they will support the bill. It is opposed by the Scottish National Party, the third-largest in the House of Commons, because it does not extend voting rights to 16- and 17-year-olds.