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Britain is considering curbing migration from some European countries in a move that would challenge a core EU principle, David Cameron said.

The PM said it was "right" to include the freedom of movement directive in a review of EU "competencies".

He also raised the prospect of a referendum on Britain's relationship with the EU after the next election - but appeared to reject an in/out vote.

And he threatened to use Britain's veto on a new EU budget "if necessary".

The comments, on the 91�ȱ�'s Andrew Marr show, come as Conservatives gather in Birmingham for their annual conference.

'New settlement'

Introducing visa controls on migrants from some European countries would challenge one of the fundamental principles of the EU.

Mr Cameron said the issue of freedom of movement would be included in a review of the UK's relationship with the EU being carried out by Foreign Secretary William Hague.

"I think it's absolutely right to look at this balance of competencies, to go through every topic and see what is in Britain's interest," he told Andrew Marr.

On the issue of a referendum, he said he wanted Britain to have a free trading agreement with Europe but not "endless political integration".

"I think the opportunity will open up, over time, to get a new settlement and when we get that new settlement, after the next election, we should have new consent for that settlement.

"And that can either take place through a referendum or, possibly, it it was close to one, at a general election."

He ruled out an in/out referendum now, as some Conservative MPs are demanding, saying: "I think the trouble with a straight yes or no, as we stand today is, frankly, I'm not happy with the status quo so I don't want to say yes to the status quo but I also don't think it would be right to leave right now."

Budget negotiations

And he took a swipe at the UK Independence Party, which some Conservative MPs fear will rob the party of an overall majority at the next election.

"I think UKIP is a complete waste of time, obviously, but I think most people in our country don't actually want to leave the European Union or just accept how it is at the moment. They want to change it."

Mr Cameron also threatened to use Britain's veto to block a new EU budget "if necessary".

He said nations could not go on "pouring" money into the EU while cutting budgets at home.

He said if "massive increases" in the budget were proposed or a deal that "does not have proper control" was on the table, he would step in.

The EU is beginning negotiations on the budget for 2014 to 2020.

Theresa May she was concerned about the impact on the British economy of a fresh wave of migration.

"We are looking at this whole area of the abuse of the freedom of movement. But we will go further on this, and the issue of free movement will be part of the review," she told the newspaper.

"It will be looking at where the decision-making powers are between the EU and the UK, how they are operating and what the impact of those are."

Mrs May did not identify the countries facing possible migration curbs in her Sunday Times interview, but the newspaper says they are understood to include Romania and Bulgaria, which are currently subject to temporary visa restrictions.