Prime Minister David Cameron warned Tuesday that Britain could leave the EU if it does not get the reforms it wants before a "once-in-a-generation" referendum to settle its troubled relationship with Europe.
In a major speech outlining Britain's demands for change following pressure from EU leaders, Cameron warned he was ready to "think again" about Britain's membership if he could not strike a deal with Brussels and the bloc's 27 other member states.
But in a sign of the British premier's looming tussle, the European Commission immediately responded, saying it deemed parts of Cameron's EU renegotiation objectives "highly problematic".
Cameron's comments came as he sends a long-awaited letter to EU president Donald Tusk laying out Britain's shopping list for change to avert a "Brexit" in a vote due to be held by 2017 at the latest.
"The referendum... will be a once-in-a-generation choice," Cameron said. "This is a huge decision for our country -- perhaps the biggest we'll make in our lifetime."
He said he had "every confidence" of securing an agreement but added that he would not rule out campaigning for a "Brexit".
"If we can't reach an agreement and if Britain's concerns were to be met with a deaf ear, which I do not believe will happen, then we will have to think again about whether this European Union is right for us," he said. "I rule nothing out."
The speech came nearly three years after Cameron first pledged a referendum on whether Britain should remain in the EU under pressure from eurosceptics in his Conservative Party and the anti-EU UK Independence Party.
Britain's turbulent ties with Brussels go back far further than the Cameron era, though.
The country joined what was then the European Economic Community in 1973 but has remained removed from the heart of Europe under successive prime ministers.
One of Cameron's Conservative predecessors, Margaret Thatcher, became an icon for eurosceptics in 1984 by securing an annual budget rebate for Britain, banging the table and demanding: "I want my money back".
Britain also notably stayed out of the euro when it was launched in 2002.
- 'Final decision' -
After Cameron won May's general election, his promise of a referendum became reality. Senior Conservatives and experts predict it will actually be held as early as next year.
Next month sees a crunch European summit in Brussels at which Britain's demands will be discussed but Cameron's Europe Minister David Lidington has played down the likelihood of getting a deal at that stage.
Cameron has said he will campaign to stay in the EU unless he cannot secure a deal which meets his demands.
Ruling out a second referendum if Britain does vote to leave the EU, he said this would be the country's "final decision" on the issue.
While Cameron's speech did not contain substantial new details about Britain's demands, it is his clearest statement yet of what he is likely to push hardest on during the negotiations.
He has long identified four broad areas where he wants to see reforms. These include improving competitiveness, greater "fairness" between eurozone and non-eurozone nations and sovereignty issues including an exemption from the aspiration of ever-closer union.
Most controversial is the demand to ban EU migrants to Britain from claiming some state benefits for four years after arriving.
- Goodbye ever closer union? -
In the speech, Cameron explicitly spoke against the central European tenet of ever-closer union, saying it was "not a commitment that should apply any longer to Britain".
He stressed that he would not be pushing for individual national parliaments to be able to veto EU measures but wants groups of national parliaments to be able to club together to do so.
On his push to restrict benefits for EU migrants for the first four years of their time in Britain, he insisted he did not want to "destroy" the principle of freedom of movement which is at the heart of the European project.
Downing Street has highlighted figures which show that 43 per cent of EU migrants rely on the support of Britain's benefits system during their first four years in the country.
Cameron's speech was greeted with scepticism by those who want Britain to leave the EU.
UKIP leader Nigel Farage said it was "clear that Mr Cameron is not aiming for any substantial renegotiation."
He added: "His speech was an attempt to portray a new 'third way' relationship with Brussels that is simply not on offer." afp