Prime Minister David Cameron has warned that Britain could leave the EU if it does not get the reforms it wants, as he laid out demands that European leaders said would be "difficult" to meet in full.
But Cameron said he was confident he could clinch an agreement to avert "Brexit" in a referendum due to be held by 2017, but would not rule out campaigning to leave the 28-member bloc if his demands are not met.
"The referendum ... will be a once-in-a-generation choice," Cameron told an audience in London on Tuesday (local time).
"This is a huge decision for our country – perhaps the biggest we'll make in our lifetime."
The address came after Cameron sent a long-awaited letter to EU president Donald Tusk, laying out Britain's shopping list for change.
While the letter contained little new detail, it helped crystallise that the main fight Cameron faces is over his plan to slash immigration by restricting state benefits to migrants from elsewhere in the EU for their first four years in Britain.
A European Commission spokesman called this idea "highly problematic" while German Chancellor Angela Merkel thought there were "some difficult [demands], others that are less difficult" while stressing she was "reasonably confident" of a deal.
Europe Minister David Lidington acknowledged "concerns" among other EU countries on the issue, adding it was the "outcome" that was important.
But eurosceptics in Britain accused Cameron of already watering down his demands after he said he was "open to different ways of dealing with" migration.
Several British newspapers reported Cameron's speech as a climbdown on the demand for a four-year benefits residency qualification, which had been a manifesto pledge of his centre-right Conservative Party.
"Cameron gives way on welfare barriers for EU migrants" read the front page of the Financial Times, while the Sun's read "the PM has wobbled on his election pledge".
Some analysts agreed. "My sense is that the line is softening," John Springford, senior research fellow at the Centre for European Reform, told AFP.
"The letter, and David Lidington's remarks, suggest that Britain will seek a compromise."
Cameron also wants reform on three less disputed matters: improving competitiveness, greater "fairness" between eurozone and non-eurozone nations, and sovereignty issues including an exemption from the EU goal of ever-closer union.
But eurosceptics reacted with scepticism to his demands.
Conservative eurosceptic Jacob Rees-Mogg called Cameron's speech "pretty thin gruel", while another, Bernard Jenkin, demanded: "Is that it? Is that the sum total of the government's position in the renegotiation?"
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."
The campaign for Britain to stay in the EU called Cameron's package of measures "sensible and sound".