The Prime Minister will today pledge an "in-out" referendum on Europe as he makes the defining speech of his premiership.
"It is time for the British people to have their say," David Cameron will declare. "It is time to settle this European question in British politics."
Mr Cameron will signal that if the Conservatives win the 2015 General Election, a referendum on Britain's membership of the European Union will take place in 2017 or 2018.
In his long-waited and much-delayed speech on Europe, the Prime Minister will say he wants Britain to stay in the EU, but it must be more "flexible and adaptable".
He will finally make the speech in central London this morning, after postponing it because of the Algeria hostage crisis only hours before he was due to make it in Amsterdam last week.
With Tory MPs and activists clamouring for a referendum and with UKIP on the march, many Conservative MPs believe today's referendum pledge could be a turning point in the battle to win the next General Election.
With its strong Euro-sceptic tone and referendum pledge, the speech is likely to delight many Tory MPs, but alarm pro-Europeans in the party like Kenneth Clarke and Lord Heseltine, who fear Mr Cameron is taking a risky gamble.
Promising a referendum, Mr Cameron will say: "The next Conservative Manifesto in 2015 will ask for a mandate from the British people for a Conservative Government to negotiate a new settlement with our European partners in the next Parliament.
"And when we have negotiated that new settlement, we will give the British people a referendum with a very simple in or out choice to stay in the EU on these new terms; or come out altogether. It will be an in-out referendum."
The Prime Minister will say legislation for a referendum will be drafted before the next election and if a Conservative government is elected it will introduce the laws immediately and pass them by the end of that year.
Confirming that the poll would take place in 2017 or 2018, he will say: "And we will complete this negotiation and hold this referendum within the first half of the next parliament."
Mr Cameron will say he speaks as a Prime Minister with a positive vision for the future of the EU, but it must change, becoming more "flexible, adaptable and open".
"If we don't address these challenges, the danger is that Europe will fail and the British people will drift towards the exit," he will warn.
"I do not want that to happen. I want the European Union to be a success. And I want a relationship between Britain and the EU that keeps us in it."
Rejecting calls for an immediate referendum, Mr Cameron will say: "Some argue that the solution is therefore to hold a straight in-out referendum now. I understand the impatience of wanting to make that choice immediately.
"But I don't believe that to make a decision at this moment is the right way forward, either for Britain or for Europe as a whole. A vote today between the status quo and leaving would be an entirely false choice.
"It is wrong to ask people whether to stay or go before we have had a chance to put the relationship right."
While Tory MPs hope the Prime Minister's speech will spark a recovery in the Conservative Party's fortunes, Labour leader Ed Miliband says it will "define him as a weak Prime Minister, being driven by his party, not by the national economic interest".
A few hours after the PM's speech, the two leaders will clash at Prime Minister's Questions in the Commons, when Mr Cameron and Tory MPs are likely to challenge the Labour leader on his policy on the EU and a referendum.