'If we vote to leave - we will leave': David Cameron's EU warning as he sets out reform demands
Video report by ITV News Deputy Political Editor Chris Ship.
David Cameron warned Britain that a decision to leave the EU cannot be reversed as he outlined his demands for renegotiation.
"If we vote to leave, we will leave - there will not be another renegotiation or referendum," the prime minister said in a speech at Chatham House.
Mr Cameron said the in-out referendum vote will be the "most important that the British people will take at the ballot box in their lifetime".
He denied that he was embarking on "Mission Impossible", saying he had "every confidence" that his demands would be accepted.
The four objectives
The Prime Minister announced his four "challenging" objectives for EU reform.
They are:
Protect the single market for Britain and others outside the eurozone
Write "competitiveness into the DNA" of the whole European Union
Exempt Britain from an ever-closer union
Tackle abuses to the right to freedom of movement and enable the UK to control migration from the EU
'Dear Donald...'
The prime minister formally outlined his demands in a six-page letter to Donald Tusk, the president of the European Council.
In the letter, he said he hoped to to get an agreement at the "earliest possible opportunity" but the "priority is to get the substance right".
Mr Tusk confirmed he had received the note and negotiations were now officially underway.
Migration and welfare
The prime minister put restricting benefits for welfare at the heart of his demands for reform.
But he hinted that he may be willing to backtrack on a proposal to ban EU migrants from claiming in-work benefits for at least four years.
Earlier, Downing Street released new figures which it says shows the extent to which EU migrants rely on the UK welfare system when they arrive.
EU migration had placed too much strain on Britain's services and infrastructure, he said, as he called for Britain to have greater control over migration.
"Ultimately, if we are going to reduce the numbers coming here, we need action that gives greater control of migration from the EU."
'Britain does not believe in ever-closer union'
The prime minister said he was calling for a "clear, legally binding and irreversible agreement to end Britain's obligation to work towards an ever-closer union".
"The commitment to ever-closer union should not apply any longer to Britain - we do not believe in it."
He added: "That will mean that Britain can never be entangled in a political union against our will or be drawn into any kind of United States of Europe."
'I rule nothing out'
David Cameron said he ruled "nothing out" if Britain's concerns about the EU "are met with a deaf ear".
The decision would be taken on a "practical" assessment of what was best for the UK, rather than any "emotional" ties to Europe.
But in a direct warning to those campaigning for Britain to leave, Mr Cameron said that choice "can't be undone".
Britain would not find itself in an economic "land of milk and honey" if it chose to leave, he said, while also stressing that it could have ramifications for national security.
"This is our only chance to get this right for Britain and for the whole European Union."
Timing of referendum
Mr Cameron said while he wanted to "get on with it", he would not announce the date of the referendum until negotiations have finished.
"The first you will here about the timing is after I have secured the the changes I need."
'I'm not going to make everybody happy'
Lord Lawson told ITV News At Ten on Monday that the prime minister had been "disappointingly unambitious" in his efforts to reform Britain's relationship with Europe.
Responding to the criticism, Mr Cameron pointed out that Britain was "far more closely tied" to Europe when Lord Lawson was chancellor in John Major's government.
He said while he would "listen respectfully to my elders and betters", he "clearly is not going to make everyone happy".
"I think I've got a track record of delivering in Europe which can help to deliver these changes," he told ITV News' Chris Ship.
Reaction
The European Commission described some of the British proposals as "highly problematic".
His political opponents, meanwhile, were quick to criticise his plans.
Ukip leader Nigel Farage told ITV News that the prime minister had not attempted to secure any sort of substantial renegotiation.
Boris Johnson, the mayor of London, said there could be "blood all over the carpet" in Brussels.
He said negotiations would be "tough" but stressed the UK could have a "very attractive" future outside the EU.
Caroline Lucas, the Green Party MP, called Mr Cameron's speech "distressing", while Labour's shadow home secretary Andy Burnham expressed disappointment at the exclusion of some items from David Cameron's "EU shopping list".
Analysis
ITV News' Chris Ship says: