LONDON (CNN) -- British Prime Minister David Cameron promised Wednesday that a vote would be held on Britain's membership in the European Union, if his party wins the next election in 2015.
Cameron said the British people should have a choice about whether to remain in the EU on the basis of a renegotiated settlement -- or to leave.
The referendum should not be held until it's clear how changes made after the crisis in the eurozone work out, he said, while giving a landmark speech on Europe.
But, he said, it was important to ask "difficult questions" about the future now. Otherwise, British people could "drift towards the exit" if Europe fails.
The key problems Europe faces are instability in the eurozone, a "crisis of European competitiveness, as other nations across the world soar ahead," and a lack of democratic accountability, Cameron said.
"There is a gap between the EU and its citizens which has grown dramatically in recent years," he said.
As a result, "democratic consent for the EU in Britain is now wafer thin," he said, and public disillusionment is at an all-time high.
"It is time to settle this European question in British politics," he said. "I say to the British people: This will be your decision."
The British prime minister insisted that he is not an isolationist, saying he would like Britain to remain part of Europe on the right terms, with the single market and competitiveness at the heart of the relationship.
And he set out a vision for an "updated" European Union that is "more flexible, more adaptable, more open, fit for the challenges of the modern age" -- and said all options should be on the table to negotiate a new deal.
He also warned that leaving the European Union would be a "one-way ticket" with no option of return.
"Over the coming weeks, months and years, I will not rest until this debate is won -- for the future of my country, for the success of the European Union and the prosperity of our peoples for generations to come," he concluded.
Cameron promised a vote in the first half of the next parliament, if his party -- which is currently in a coalition government with the Liberal Democrats -- wins the general election due in 2015.
The prime minister has been under concerted pressure from some members of his party to hold a referendum on Europe. The so-called Eurosceptics want to see certain powers repatriated from the EU to Britain and a reduction in legislation they say holds back businesses.
However, others both in and outside the party warn that creating uncertainty about Britain's membership in the European Union risks undermining the confidence of businesses and industry at a critical time.
Fellow European nations may also be unwilling to renegotiate the terms of Britain's membership.
France and others have made it clear that Cameron cannot "cherry-pick" which elements of the European Union he signs up to, or risk unraveling the union to suit British interests.
French Foreign Minister Laurent Fabius told radio station France Info that the planned referendum "risks being dangerous for Britain itself because Britain outside of Europe, that will be difficult."
If Britain did decide to leave the European Union, France would roll out the red carpet for business leaders, Fabius added.
U.S. President Barack Obama last week told Cameron that "the United States values a strong UK in a strong European Union."
Markets across Europe were flat Wednesday morning after Cameron's speech, including in London.
Labour leader Ed Miliband said ahead of Cameron's address that it would "define him as a weak prime minister, being driven by his party, not by the national economic interest."
Miliband said Tuesday that Cameron had rejected the idea of an "in/out" referendum on Europe in October 2011 because of the doubts it would foster.
"The only thing that has changed since then is he has lost control of his party and is too weak to do what is right for the country," Miliband said.
"Everyone knows that the priority for Britain is the jobs and growth that we need. We have had warning after warning from British business about the dangers of creating years of uncertainty for Britain."
Daniel Wilsher, an expert in EU law at City University London, said Cameron will need to come up with "some convincing bargaining chips" if he wants to persuade other EU members to negotiate a new deal.
Cameron's threat to wield Britain's veto over treaty changes to deepen ties within the eurozone will not be enough, Wilsher said.
The British prime minister also focused on the importance of the single market, Wilsher said, but membership in this is tied to EU rules regarding workers' rights, consumer safety and environmental protection.
To opt out of certain laws, such as the EU directive on working hours, "is legally difficult and would disturb a premise of the single market," he said. "Overall, it would be very hard to change the fundamentals without allowing every country to pick on its own pet hates."
Cameron was due to give the speech on Europe last week but had to postpone its delivery because of the Algerian hostage crisis.
-- CNN's Claudia Rebaza contributed to this report.
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