LONDON — Prime Minister David Cameron promised Britons a decisive referendum within five years on membership in the European Union — provided he wins the next election — in a long-awaited speech on Wednesday whose implications have alarmed the Obama administration and are likely to set the markers for an intense debate in Britain and across Europe.

“It is time for the British people to have their say. It is time to settle this European question in British politics,” he told an audience in London, raising fears in capitals as distant as Washington that a ballot could lead to Britain’s withdrawal from the European Union.  Read more