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.
Cameron told employers' organization the Confederation of British Industry that he wants a looser, "more competitive" EU that gives greater protection to countries, including Britain, that don't use the shared euro currency. The EU has 28 members, 19 of whom use the euro.
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, leader of Britain's Conservatives, 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 to 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 says he will publish his negotiating demands Tuesday in a letter to European Council President Donald Tusk. He said he was seeking "a live-and-let-live Europe, a flexible Europe" that lets Britain remain outside the closer political and economic ties binding the eurozone countries.
"We want to be in a common market, not a common country," he said Monday.
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?"