BRUSSELS — The longest-serving government leader in the European Union, Jean-Claude Juncker of Luxembourg, bowed Wednesday to outrage over revelations of a litany of abuses by his tiny country’s wayward intelligence service and declared that he would step down as prime minister.

Although he is from a country with only 539,000 residents, Mr. Juncker, 58, has become one of Europe’s most high-profile political figures in recent years because of his previous role as president of the so-called Eurogroup of finance ministers as they struggled to contain a rolling debt crisis and hold the common currency together. His tenure in that position ended in January.

Mr. Juncker, who became prime minister in 1995, has not been accused of any criminal wrongdoing but has come under attack for not keeping the security service in check. Mr. Juncker’s demise was an unlikely one — a tumultuous political drama set off by intelligence service skulduggery in a deeply conservative country known for its secretive banks and deep-rooted traditions of settling disputes behind closed doors.

Mr. Juncker, whose Christian Social Party has governed Luxembourg almost continuously since the end of World War II, pledged to step down during a hearing in Parliament on a report commissioned by the assembly on the activities of Luxembourg’s intelligence service, known as SREL, after its French acronym.