Prime Minister David Cameron said Tuesday that the vote on the UK’s membership of the European Union is the most important decision the country will face for a lifetime.

The UK will hold a referendum on its continued membership of the European Union before the end of 2017, with recent speculation saying it could come as early as September 2016.

Cameron’s speech comes alongside the publication of a letter to Donald Tusk, the President of the European Council, setting out the UK’s demands for a renegotiated membership, with the British government to hold that referendum once any changes are agreed.

Recent polls have suggested that Britons may well vote to leave if Cameron fails to get acceptable new terms from Brussels.

Speaking at Chatham House in London, Cameron said that the the UK is not subscribed to the idea of “ever closer union”, adding that national parliaments should have a greater say in EU lawmaking.

“The renegotiation is now entering its formal phase following several rounds of technical discussions… this is perhaps the most important decision that the British people will have to take at the ballot box in our lifetimes,” he said.