LONDON — With speculation growing about Britain’s future in the European Union, Tony Blair, the former prime minister, warned on Wednesday that the country faced a “real and present danger” if it edged toward leaving its regional alliance while power shifts to emerging economies.

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

Britons have rarely shown much enthusiasm for the idea of European integration but, in recent months, discussion over Britain’s relationship with — or even membership in — the European Union has intensified, with so-called Euroskeptics in the ascendancy.

Addressing a business lobby group, Mr. Blair said Europe was an “absolutely essential part of our nation remaining a world power,” and it would be a “monumental error of statesmanship to turn our back on it,” he added.