David Cameron has denied being forced to back down over his high-profile European treaty veto.
The Prime Minister was derided by Labour after it emerged that key EU institutions could be used to enforce the new fiscal pact.
But he insisted the arrangements between 25 member states would not damage Britain's national interest - and the Government could take legal action if it did.
The UK blocked a full treaty of all 27 countries setting rules for eurozone fiscal discipline at the December summit of the European Council. The Czech Republic has joined Britain in saying it would not sign up to the deal, expected to be finalised in March.
As he reported back to the Commons on the outcome of the latest summit, the premier faced mockery from Ed Miliband.
Mr Cameron had originally promised that EU institutions such as the European Commission and European Court of Justice would not be allowed to support the new agreement, the Labour leader said. "With this Prime Minister, a veto is not for life, it's just for Christmas," he joked.
"He said it was a real veto on the use of EU institutions and his backbenchers believed him, even his Cabinet believed him. On the European court, on the Commission, on the buildings, the phantom veto of December is now exposed."
Mr Miliband also rejected Mr Cameron's claims that there was no new European treaty because he had vetoed it. "It talks like a European treaty, it walks like a European treaty, it is a European treaty," he said. "And for Britain he has secured absolutely no protections at all."
However, Mr Cameron said some use of the institutions by the group of 25 was already permitted by existing EU treaties.
"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 misuse of the institutions," he added. "The principle that the EU institutions should only act on the explicit authorisation of all member states remains."