LONDON (AP) — The European Union will have to become more flexible if it wants Britain to stay in the 28-nation bloc, Prime Minister David Cameron said Monday, as he insisted he is "deadly serious" about leaving if he doesn't get his way.
The U.K. government will publish its list of demands for negotiation with other EU nations on Tuesday, and is seeking to impress upon its neighbors that a "Brexit" is a real possibility.
"The argument isn't whether Britain could survive outside the EU —of course we could," Cameron said. "The argument is, how are we going to be best off?"
Britain, which has long had an ambivalent relationship with the EU and its vision of a borderless Europe, will hold a referendum by the end of 2017 on whether to remain or leave.
Cameron says he wants to stay in — provided he secures changes allowing Britain greater autonomy.
So far, his goals have been short on detail, beyond a general desire to control immigration by limiting welfare benefits for new arrivals and to ensure that the 19 eurozone countries can't impose measures on non-euro members.
Other EU nations have been broadly sympathetic, but say they want to see concrete proposals from Britain.
Cameron will publish his negotiating demands Tuesday in a letter to European Council President Donald Tusk. His office said Monday that Britain would seek reform in four areas — "sovereignty, economic governance, competitiveness and migration/welfare."
Cameron is also due to give a speech in London about the demands on Tuesday morning.
Cameron told employers' organization the Confederation of British Industry on Monday that he wants a looser, "more competitive" EU that gives greater protection to countries, including Britain, that don't use the euro currency shared by 19 EU nations.
He said he was seeking "a live-and-let-live Europe, a flexible Europe."
"We want to be in a common market, not a common country," he said.
Cameron faces the tough task of making the pro-Europe case while facing down the many anti-EU members of his Conservative Party, who believe he's not serious about leaving the bloc.
He was briefly heckled Monday by Euroskeptic students who held up a banner calling the CBI group "the voice of Brussels." Many business interests in Britain favor remaining in the EU, fearing wide disruption for U.K. manufacturers and for London's financial industry if the country leaves the EU's 500 million-person common market.
Cameron insisted "people in Europe know I am deadly serious."
"If it's flexible enough, we'll stay," he said. "If it's not flexible enough, we will have to ask ourselves a very profound question: Is this organization for us?"
He insisted that Britain, as the world's fifth-biggest economy, could survive outside the EU.
That view was echoed by Foreign Secretary Philip Hammond, who is visiting the United Nations in New York.
"If we had to operate outside the European Union, we would do so," Hammond told reporters. "It would be challenging. We would have to rethink the way we operated, but it is not impossible."
Associated Press writer Edith M. Lederer at the United Nations contributed to this report.