Cameron's pledge on EU referendum
David Cameron has stressed that his commitment to holding an in/out referendum on European Union membership will be a red line in any coalition negotiations following the 2015 general election.
The Prime Minister said he will not lead a government unless it implements his plan for a vote by the end of 2017 on severing ties with Brussels.
Mr Cameron said he did not believe the British people wanted to "instantly get up and walk away" from Europe but support for the current situation had "worn wafer thin".
He said the commitment to a referendum was not something that he would "barter away" in any negotiations should there be a hung parliament after the 2015 election.
The Prime Minister was taking part in a telephone conference aimed at Tory supporters ahead of May 22's European elections, where the Conservative vote risks deserting the party for Nigel Farage's Ukip.
Mr Cameron said the issue of Europe would be one of his main concerns if he remained in No 10 after the next general election.
He said: "It's very high up the priority list. If the Conservative Party and I am re-elected in 2015, which I believe we can do - it's going to be tough, it's going to be a big battle, but we can do it - if I achieve that in 2015, then we would have to work quickly to complete that renegotiation and hold that referendum.
"So it would be very high up the list of priorities for an incoming government.
"And as I've tried to say, this is not something that I would ever barter away or give away. I would not be prime minister of a government unless we could carry out our pledge of an in/out referendum.
"I think that's a really clear promise and a very clear statement to make."
But taking calls from party supporters and members of the public, Mr Cameron's ability to deliver changes to the EU was questioned.
But one Conservative activist and small businessman from London, told him: "I don't doubt your sincerity about the commitment to have a referendum, I just don't think you are going to be allowed to negotiate sufficient changes to satisfy the British public and people like me."
He added: "They are simply not going to give you sufficient room to change the rules just for the UK."
But Mr Cameron stressed there was support for Britain's continuing membership from other countries, including the EU's economic powerhouse Germany, which strengthened the chances of a renegotiation.
He said: " By and large, particularly the countries of northern Europe, they want Britain as part of the European Union. They recognise that consent in Britain has worn wafer thin for membership of this organisation.
"The Germans, the Danes, the Dutch, the Swedes - they want countries that support free trade, that support enterprise, like Britain, that are a force for good in the European Union.
"The second point is the eurozone countries need change. Their currency is creaking in terms of the changes it needs in the European treaties.
"If they need changes to help sort out the eurozone, there's no reason why we shouldn't have changes to sort out those issues that Britain thinks have gone wrong in Europe.
"The third reason is we are being clear about the things that we need: ever-closer union should not apply to the UK, benefit tourism needs to end, future members who join the European Union should not be given instant access to Britain's labour markets, we do need to deal with things like the working time directive which has caused so much difficulty in our NHS.
"We have set out the sorts of things we want to negotiate, I think they are achievable, doable, deliverable. I say that because I have had discussions with other European politicians.
"Of course, they would rather that Britain didn't want any changes. But the fact is they want Britain as part of the EU, they need changes themselves, we are asking for sensible changes that can be delivered and I am confident.
"I wouldn't have set out down this path if I didn't think I could deliver a good deal for Britain and a referendum. I can deliver both of those things, they are the right things for Britain and they happen to be what the British people want.
"I don't think the British people want instantly to get up and walk away from Europe, nor do the British people want to just accept what we get given from Europe.
"They want change and I think increasingly they can see that this Government is capable of delivering change - not least because we have done things like cut the budget that people didn't think we would be able to do."
In his message to activists, Mr Cameron said US-style technology-led campaigning could not replace the need for people to knock on voters' doors.
"People are cynical about politics and politicians and what can be achieved and what can be changed," he said.
"Nothing works better than personal contact between one of our activists and a voter on the doorstep.
"People talk about all the extraordinary things that the American election proved about technology, and there's some truth in that, but in the end all the technology was designed to try and deliver an activist on the doorstep talking to a voter and convincing them about the candidate and about the policy.
"That's what we've got to do."
Matthew Elliott, chief executive of Business for Britain, said: "The Prime Minister is right to make a referendum a red line for any future government of his, especially if he wants British business to prosper in years to come.
"Britain will only be set free from the status quo and get a better deal from the EU if negotiations for reform are backed up by the guarantee of a referendum.
"Those in Westminster who talk the talk of reform must now walk the walk and commit to giving Britain a vote in 2017 - just as polling shows British business wants."
Shadow cabinet minister Michael Dugher - who along with Labour strategy director Greg Beales makes up the party's negotiating team - said the Prime Minister was scared of entering the fray.
"I can see why Cameron doesn't fancy it; he is like a fighter with a glass jaw with his record of failure, broken promises, standing up for the wrong people.
"But that's not good enough," he told the BBC Radio 4 Today programme.
"Ed Miliband has made his position absolutely clear about the importance of the TV debates, about his determination to get on with them. We are all waiting for Cameron.
"We know why he is not keen but frankly these should not be in his gift. They belong to the public and we should get on with them."
Asked whether Mr Cameron would quit as Prime Minister if Scotland votes for independence later this year, the PM's official spokesman said: "With regard to the Scottish referendum, what is on the ballot in September is the future of the people of Scotland not the future of individual politicians."