David Cameron has set out the red lines for any future coalition negotiations by issuing an "absolutely" cast-iron guarantee that he will stage a referendum on Britain's membership of the European Union.
The Prime Minister insisted he will not return to No 10 unless he can secure an agreement that the in/out vote will go ahead in 2017.
As voters prepare to go the polls for the European Parliament elections later this month, the Conservative leader insisted he is "confident" he could renegotiate the UK's relationship with Brussels.
But he pledged the referendum would go ahead even if his plan to bring back powers first fails, and said he would not " sit on the fence" over whether to recommend voting Yes or No in the event of an unsuccessful bid.
Speaking on the BBC's Andrew Marr Show, he said: "I've said very clearly that whatever the outcome of the next election, and of course I want an overall majority and I'm hoping and believing I can win an overall majority, but people should be in no doubt I will not become Prime Minister unless I can guarantee that we will hold that referendum.
"I won't become Prime Minister unless I can guarantee that that in/out referendum will be held."
Asked if he could give a "cast-iron guarantee" that any government he led would stage a referendum within two years, the PM replied: "Absolutely."
He added: "We have the referendum whether or not I have successfully negotiated."
Mr Cameron said the "most important" change he wanted to secure was reform to get Britain out of a clause on an ever closer union.
He also wants to renegotiate powers over border control, crack down on benefit tourism, secure more trade and have greater control over justice and home affairs.
"I would like to see longer periods in terms of before you are allowed to claim any benefits," he added.
"Let's look at what is possible but certainly what is longer than what we have today."
The Prime Minister said free movement within the EU is "important", but insisted it "needs to be returned to the original concept, which was the freedom to be able to go and work in another country".
Polling has indicated the UK Independence Party will come first in the UK's European Parliament results, with the Conservatives pushed into a humiliating third place.
Ukip leader Nigel Farage rubbished Mr Cameron's promises, insisting voters will struggle to believe the Tory leader's claims because he "didn't deliver" on a previous pledge to hold an EU referendum.
He told BBC One's Sunday Politics: "He has difficulty being believed because five years ago I sat here talking to you about the European elections and you put to me 'well it's going to be very difficult Nigel for people to vote Ukip because Mr Cameron has given a cast-iron guarantee that if he becomes Prime Minister next year he'll have a referendum on the Lisbon Treaty'.
"He didn't deliver on that and he knows that people struggle to believe firstly that he will get a majority, and secondly that the renegotiation is actually worth a row of beans."
Mr Farage added: "He is desperately trying to pretend to be a Eurosceptic whilst at the same time saying whatever the results of all of this he will campaign for Britain to remain in."
Shadow foreign secretary Douglas Alexander said: "David Cameron has now admitted that his negotiations with Europe could fail, but he won't accept the consequences of what this could mean for Britain.
"He can't say what reforms he wants to deliver, he can't say what he would do if he isn't successful, and now we know he can't even say how he will vote in a referendum.
"David Cameron seems willing to let the UK sleep-walk out of Europe altogether simply to stop his Eurosceptic backbenchers walking out on him.
"He is now too weak to deliver real change in Europe, because instead of trying to negotiate with other European leaders, he's spent too much time negotiating with his own backbenchers."