LONDON - Prime Minister David Cameron moved Monday to beat back a brewing rebellion within his Conservative Party over Britain's membership of the European Union, rejecting demands for a speedy public vote on exiting the trading bloc but insisting that he would offer such a referendum by the end of 2017.

As he met President Obama in Washington - where Cameron lobbied for a U.S.-EU free trade pact - the British leader found himself on the defensive at home against members of his own party who want their country to withdraw from the 27-nation EU as soon as possible.

Dozens of "Euroskeptic" Conservative lawmakers are backing a measure in Parliament on Wednesday expressing regret over Cameron's failure to commit to legislation that would bind the government to calling a plebiscite. Although the proposal has little chance of passage, the criticism from his own side has heaped pressure on Cameron over an issue that threatens to convulse his party and endanger his leadership of it.

Adding to his woes were comments from two of his own cabinet ministers who said over the weekend that they would vote for Britain to leave the European Union if an immediate referendum were held. And the startling success of an anti-EU group, the UK Independence Party, in local elections earlier this month, largely at the expense of Conservative candidates, has raised the stakes.

Cameron, who favors continued EU membership as long as Britain can reclaim some powers from Brussels, seemed irritated by the open defiance from his backbenches.

"There's not going to be a referendum tomorrow," he told reporters. "There is going to be a referendum before the end of 2017, and between now and then, the task is to renegotiate our position, to reform the European Union, to put a real choice to the British people."

He accused dissidents of "throwing in the towel" on the EU before waiting to see the outcome of Britain's negotiations with its European partners.

Cameron knows he must tread carefully on the issue of Europe, which has been a minefield for Tory leaders. The party's two previous prime ministers, John Major and the late Margaret Thatcher, were both toppled from power in part because of domestic controversies over Britain's vexed relations with its continental neighbors.

Some polls show more Britons in favor of quitting the EU, the world's largest trading bloc, than staying in; many here chafe against what they see as overregulation from Brussels and a loss of national sovereignty. Yet Cameron must also hearken to the pro-EU sentiments of the Liberal Democrats, his coalition partners in government, and of big businesses, a key Conservative constituency.

The opposition Labor Party has savored the discord across the aisle, describing it as a reflection of a prime minister unable to exert his authority over the rank and file.