David Cameron has vowed to campaign "with all my heart and soul" for continued British membership of a reformed EU in an in/out referendum which will be staged by 2017 if Conservatives win the next general election.
The Prime Minister was cheered by Tory backbenchers as he arrived at the House of Commons after announcing that his party's election manifesto will seek a mandate to negotiate a "new settlement" for Britain, which will be put to voters in a referendum by the mid-point of the next Parliament.
But the plan brought divisions within the coalition government to the fore, as Liberal Democrat Deputy Prime Minister Nick Clegg said renegotiation was "not in the national interest" and would create damaging uncertainty for business.
Labour leader Ed Miliband said that jubilant Conservatives wanted Britain out of the EU, and accused the Prime Minister of taking a "huge gamble" with the economy because he was "running scared" of the UK Independence Party (Ukip) and his own backbenchers.
But the referendum promise heaped pressure on Mr Miliband, who appeared in the Commons to rule out Labour offering the public a vote. Shadow foreign secretary Douglas Alexander later sought to clarify his leader's stance, insisting Labour had "never said never" to a referendum, but did not think it was right to promise one now.
In a long-awaited speech in London, Mr Cameron said he wanted a new treaty to reform the EU for all its members, but was ready to demand a renegotiated status for Britain alone if other nations did not agree.
Draft legislation will be drawn up by the Conservative Party ahead of the election, and will be enacted by the end of 2015 if Tories win to pave the way for renegotiation and referendum within the next two years, he said.
"It is time for the British people to have their say. It is time to settle this European question in British politics," Mr Cameron said.
But there were immediate questions over whether other EU states would be prepared to agree special terms for the UK.
German Chancellor Angela Merkel said she was ready to find a "fair compromise", but cautioned that Britain must recognise that other countries would have wishes of their own.