The blueprint for reform of the European Union is a "fair" settlement for Britain, the president of the European Commission (EC) has said.
Jean-Claude Juncker said plans for an emergency brake would address the "record numbers" of EU citizens that moved to the UK after the government failed to apply transitional measures more than decade ago.
It would last for a "period of up to four years" and would apply in "exceptional cases", he said.
"The duration of the mechanism will be limited in time. That is a crucial characteristic of a safeguard mechanism necessary in order to make it compatible with the treaties."
He added: "In effect we will enable the United Kingdom to use the mechanism to address the consequences of that decision."
Juncker said he had worked hard with British Prime Minister David Cameron to shape the deal, which addresses issues including unchecked migration and welfare benefits for new arrivals.
The EC supported the proposals, he added.
"The settlement that has been proposed is fair for the UK and fair for the other 27 member states."
The UK already benefits from more opt-outs and protocols than any other member state, he told the European Parliament.
"This is why, as a matter of law and a matter of fact, the concept of ever closer union has already assumed a different meaning in its case," he added.
"The settlement recognises this. It recognises that if the United Kingdom considered that it is now at the limits of its level of integration then that is fine. At the same time, it makes clear that other member states can move towards a deeper degree of integration as they see fit.
"In this way, we have addressed the prime minister's concern, while respecting the treaties."
Juncker said it was legitimate for member states outside the eurozone not to have financial responsibility for measures taken to shore up the currency bloc.
"The euro remains the currency of the European Union, the parliament remains the parliament of the union as a whole."
Negotiations with the UK, along with the refugee crisis, have allowed commentators to claim there was a "psychodrama" unfolding where Europe is "falling apart little by little", he added.