David Cameron risked a further rift with his own backbench MPs as he dismissed calls for a referendum on the UK's membership of the European Union (EU).
The Prime Minister insisted he shared the views of eurosceptics in his party but said he was "optimistic" that the UK's relationship with Brussels was changing to meet the concerns of British people.
He acknowledged the closer links forged by eurozone countries would have consequences for the UK but said no new powers would be passed from Westminster to Brussels.
Nearly 100 Conservative MPs have written to Mr Cameron urging him to make it a legal commitment to hold a poll on the UK's relationship with the EU during the next parliament.
Speaking at a press conference following the latest EU summit in Brussels, Mr Cameron said: "I completely understand people's concerns and I share a lot of the concerns people have."
But he said it was "vitally important" that the Government had put in place measures which meant new powers could not be passed to the EU without a referendum.
He also questioned the timing of any referendum on the UK's membership given the evolving situation on the continent.
He said: "Europe is changing, there is a change taking place as the countries of the eurozone follow the remorseless logic of having a single currency but having 17 different finance ministries and all the rest of it.
"They need to change and they recognise that.
"That change has consequences for Britain. My job is to make sure we secure all the safeguards that we need so that our role in the European Union, our access to the single market, our say in the single market, is properly safeguarded.
"But this is going to be something that is going to evolve over a whole series of years as these countries realise what needs to be done and as we fight for the safeguards and the position we need."
He added: "Europe is changing, Britain is not going to cede more powers to Brussels and I have often argued I think there are powers that should be going in the other direction.
"This is going to be an unfolding story but one where I think Britain has every chance of securing the sort of relationship that we want in Europe."
He also said a straight referendum on whether to leave the EU would not meet the concerns of many Britons.
Mr Cameron said: "I completely understand why some people want an in/out referendum, why they wanted it yesterday, why they want it today,
"Some people just want to get out: 'stop the bus I want to get off'.
"I completely understand that but I don't share that view, I don't think that's the right thing to do.
"The problem with an in/out referendum is it actually only gives people those two choices: you can either stay in, with all the status quo, or you can get out.
"Most people in Britain, I think, what they want is a Government that stands up and fights for them in Europe and gets the things we want in Europe, that changes some of the relationship we have in Europe."
He added: "I think that the trouble with the argument about the in/out is it's only those two options, whereas I think what we want is a Government that stands up and fights for Britain in Europe, gets what we want."
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The Prime Minister described himself as a "practical" Eurosceptic but said he was optimistic about getting the UK's relationship with Brussels right.
"For those of us who are practical Eurosceptics, who know there is a real benefit from being engaged but are frustrated by some of the ways the relationship works, I see lots of reasons to say the argument is going in our direction," he said.
"I am not only a practical Eurosceptic, I am an optimist about getting this relationship right."
Mr Cameron insisted he was not at loggerheads with his backbenchers over Europe, saying: "I completely share their view.
"I think Conservative MPs, broadly speaking, are very happy we have got the lock, because you know with the referendum lock - to put it brutally - that things can't get worse.
"You can't have a Government that suddenly pitches up and says 'Oh, sorry, we've signed another treaty and a whole lot of powers are going from Britain to Brussels'.
"That happened at Nice, it happened at Amsterdam and Lisbon, and it was immensely frustrating for people who said 'Hold on, we are elected to help govern the country and yet the rules of the game keep changing and the British public aren't asked'.
"I think Conservative MPs and, I would argue, people across the country are pleased that we have this lock."
He added: "The question for the future is how is Europe going to change and how is Britain's relationship with Europe going to change as Europe changes?
"I am actually confident that Britain, fighting and standing up for itself in Europe, can secure good deals in Europe - as I think this European summit has shown."