The EU council president says the package of reforms discussed with British Prime Minister David Cameron cannot be reversed by European judges.
Addressing MEPs, Donald Tusk said the deal was "legally binding and irreversible."
The remarks come after British Justice Secretary Michael Gove said that the European Court of Justice could throw out some measures without EU treaty change.
Both Downing Street and attorney general Jeremy Wright say the reforms cannot be reversed.
A UK referendum on EU membership is set to take place on 23 June, with the Conservative Party and David Cameron's cabinet divided over which side to support.
Gove is one of five cabinet ministers campaigning for an EU exit. He said that without treaty change all elements of the PM's renegotiation settlement were potentially subject to legal challenge.
"The facts are that the European Court of Justice is not bound by this agreement until treaties are changed and we don't know when that will be," he said.
But Cameron’s spokesman said the text of the deal would be deposited at the UN adding this would "put beyond doubt that it is legally binding and irreversible in EU law."
Tusk said it was "in conformity with the treaties and cannot be annulled by the European Court of Justice."