David Cameron has dismissed claims that his EU reform package could be undone in the future even if it is agreed at a crucial Brussels summit later this month.
The president of the European Parliament, Martin Schulz, sparked fresh controversy over the deal hammered out by the Prime Minister when he warned: "Nothing is irreversible".
However, speaking in Copenhagen, Mr Cameron said that in practice the package could not be undone once it has been agreed by all 28 member states.
"If it is agreed it will be agreed as a legally binding treaty deposited at the United Nations," he said.
"It would only be reversible if all 28 countries including Britain agreed to reverse it.
"Given that it's the treaty that Britain wants, there is no way we are going to agree to reverse it.
"So while you can argue that it is technically reversible if we agree to reverse it, it is not in fact reversible."
His comments came after Mr Schulz told Sky News: "Nothing in life is irreversible, therefore legally binding decisions are also reversible, nothing is irreversible."