David Cameron has denied watering down his EU renegotiation demands as he insisted he was "deadly serious" about the need for reform.
The Prime Minister, whose speech to the CBI was briefly disrupted by Eurosceptic hecklers, said he was ready to dive "headlong" into making the case for Britain to stay in the union.
But he said that would only be possible if he secured changes including barring EU migrants from claiming tax credits and child benefit until they have lived in the UK for at least four years.
Mr Cameron - who will set out his main renegotiation aims in a letter to European Council president Donald Tusk on Tuesday - said he had been using the past few weeks to "debunk" arguments about the EU that he regarded as "duff".
He has already dismissed the idea that the UK would benefit from a Norway-style semi-detached relationship with the union, and said he was tackling suggestions that the country could not survive outside the EU.
"When it comes to the crucial issues, our prosperity, our national security, of course we could try to look after those things outside the EU, but how do we make ourselves more prosperous and secure?" he told business leaders.
"We need to fix these challenges, fix these problems. That is what the renegotiation is about and then we can throw ourselves headlong into keeping Britain in a reformed Europe."
He added: "I don't have any emotional attachments to the institutions of the EU. But I have a very strong emotional and practical attachment to asking the simple questions for Britain: How do we have more influence in the world? How do we have more prosperity? How do we have more jobs? How do we do the best for this country that we love? To me that is what it is all about."
Mr Cameron said other EU leaders should be in no doubt that he was "deadly serious" about the UK's membership being in question.
"I couldn't have been more clear with my colleagues," he told the conference. "I have been to every single president and prime minister and I have very patiently set out what needs to change.
"But patiently setting out a list of very sensible changes should not be mistaken for a lack of resolve.
"If these things can't be fixed then Britain would naturally ask, 'Do we belong in this organisation?'.
"I think people in Europe know I am deadly serious about that and that is what the negotiation that we will be launching tomorrow is all about."
He went on: "On the migrant issue, we have not changed our view at all. We believe that what is set out in the manifesto is right for Britain, right for Europe and needs to change."
The Conservative general election manifesto said: "We will insist that EU migrants who want to claim tax credits and child benefit must live here and contribute to our country for a minimum of four years."
However, there has been speculation that Mr Cameron is ready to drop the demand, as other EU leaders regard it as breaching the principle of free movement of labour.
As the Prime Minister delivered a speech to the CBI conference in London, two teenagers stood up holding a banner and chanting: "CBI - voice of Brussels."
The PM said: "Come on, guys. If you sit down now, you can ask me a question rather than making fools of yourselves by just standing up and protesting."
The pair, Phil Sheppard and Peter Lyon, both aged 19, were quickly ushered out by security.
The protesters told Sky News outside the conference that they had been helped by the Vote Leave campaign group and were granted valid passes by the CBI.
"We got in because Vote Leave formed a company for us that was able to get us in. We managed to get our passes and we pretended to be businessmen," Mr Sheppard said.
Mr Lyon described the experience as "terrifying" but insisted it had been "worthwhile" to highlight concerns.
"We were worried about the CBI misrepresenting British business's views ... in reality a lot of particularly small and medium-sized businesses in Britain think the EU hinders rather than helps them," he added.
A CBI spokesman said: "The CBI speaks on behalf of 190,000 firms of all sizes, employing seven million people, in every sector and in every corner of the country.
"We have always said that there is no uniform business view on the EU referendum inside or outside the CBI, but a majority of our member companies want the UK to be in a reformed EU.
"It is disappointing that some individuals have decided to disrupt the enjoyment of the conference for delegates."
Members of a trade union group opposed to the EU also staged a demonstration against the CBI, but from a grassed area opposite the hotel in central London where the conference was being held.
CBI director general John Cridland said: "The majority of CBI members want to be in a reformed EU, but we've been clear that the EU is far from perfect and needs to be more focused on growth, jobs and trade. That includes tackling areas of burdensome lifestyle and employment regulation.
"Further integration in the eurozone isn't something the UK wants to be part of. When the Prime Minister comes back with his EU reform package, the CBI will consult its members again."
Shadow foreign secretary Hilary Benn said: "It is important that the way in which the European Union works changes to reflect a changing world and Britain's priorities.
"But we also have to say clearly that Britain is stronger and more successful because we are part of the EU."