United Nations: Britain has registered at the United Nations the agreement that it reached last week to try to keep the United Kingdom in the European Union.

David Cameron. Pic/ AFP

British Ambassador Matthew Rycroft on Wednesday said under the UN Charter, member states have to register decisions that are at the level of a treaty. "That puts beyond doubt that the deal that the PM agreed is legally binding and irreversible in international law," he said.

The agreement was key to Britain’s announcement of a referendum on June 23 on whether Britain should remain in the EU or leave the 28-nation bloc. Prime Minister David Cameron said the deal with the EU would give Britain more control over its future, lessen welfare payments to migrants, and protect Britain from being absorbed into a feared European "superstate."

Rycroft said there is an issue about when the agreement enters into effect. "But officially it has entered into force now, and it has effect on June 23 if the UK votes to remain in the EU," he said.