LONDON — With power shifting toward the world’s emerging economies, Britain faces a real and present danger if it edges toward the exit of the European Union, its regional alliance of 27 nations, Tony Blair, the former British prime minister, warned Wednesday.

Speaking to a business lobby group in London, Blair entered the fraught debate about his country’s place in the union with a speech arguing that the bloc was more important now than ever because it helped Britain leverage its influence within a changed geopolitical landscape.