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David Cameron has defended his decision not to block the use of EU institutions by a new eurozone fiscal pact.

He told the Commons the pact "places no obligations" on the UK, but he would take "legal action" if the future role of the European Court threatened UK.

Conservative MPs have accused the PM of weakening the stance he took last year when he vetoed EU treaty change.

And Labour leader Ed Miliband called it a "phantom veto" and said the PM had secured "no protections" for Britain.

Mr Cameron vetoed an outline fiscal pact in December, saying the UK had not got sufficient safeguards over the future workings of the single market and financial regulation affecting the City of London.

The Czech Republic now has joined the UK by saying it will not sign the agreement but 25 other EU states have agreed to join up.

In December the PM also suggested that the European Court of Justice and the European Commission must remain for the use of all 27 EU states, and not for the fiscal pact countries alone.

But he has now said he will not veto any move to use the court to enforce the accord.

Giving a statement to MPs following a summit in Brussels on Monday, Mr Cameron said there were "legal questions" about the use of the court, but the wording of the new accord was "absolutely explicit and clear that it can't encroach on the competencies of the European Union".

"But nevertheless I made clear we will watch this closely and if necessary we will take action, including legal action, if our national interests are threatened by the misuse of the EU institutions," he said.

Earlier, Mr Miliband said the prime minister had "sold us down the river on a lot of things" in Europe.

Replying to Mr Cameron in the Commons, the Labour leader said the PM's "phantom veto" had left Britain isolated in Europe without any of the safeguards discussed in December.

"With this prime minister, a veto is not for life, it's just for Christmas," he said.

"He said there isn't an EU treaty, because I vetoed it - it doesn't exist. So this agreement involves the European Court of Justice, the European Commission, the European buildings, 25 out of 27 countries, and he says it's not really a treaty."

Former Labour foreign secretary Jack Straw asked the prime minister: "What has been achieved by your veto except that we are outside the door?"

The Liberal Democrats had criticised the December veto, but several senior party members gave their support to Mr Cameron for this latest development.

Sir Menzies Campbell praised the "policy of re-engagement" adopted by the PM, while Simon Hughes said Monday's summit had been "much more successful" than the previous one.

But Mr Cameron was pressed by his own Eurosceptic backbenchers for reassurance - although they stopped short of directly criticising him.

Bill Cash asked whether the UK was on "a slippery slope" and the fiscal pact could eventually be "folded into the EU treaty".

Mr Cameron said it "cannot be folded back into the EU without the agreement of every member state", and because he had vetoed a new EU treaty "we're not in danger of that happening".

Bernard Jenkin questioned why "a subset of member states can by-pass a veto and hijack the institutions for their own purposes".

The PM said there were already groups within the EU that used the institutions and the UK "had the ability to exercise leverage" to ensure the new fiscal pact did not exceed its remit.

Several Eurosceptic Conservatives said earlier that there should now be a full referendum on UK membership of the EU.

Douglas Carswell suggested the "cast-iron guarantees" apparently won earlier had "rusted away", and Europe was "back to business as usual - ministers and mandarins cutting deals, the people of this country ignored".

Senior Tory backbencher David Davis told ����ý Radio 4's World at One that the European Court had "a history of expanding its brief and applying principles in the treaty to areas where nobody thought they would".

"We've suffered from it in the past in the fishing industry, and there's a real risk that by allowing the court to have a say on the implementation of the new fiscal union then we will actually do ourselves harm, without realising it, in the future," he said.

A Downing Street spokesman said earlier there had been "agreement round the Cabinet table" when the PM reported back on the outcome of the summit on Tuesday.

Work and Pensions Secretary Iain Duncan Smith raised legal issues over the use of EU institutions, the spokesman said, but "recognised" that they had been "registered in the treaty".