LONDON — Raoul Wallenberg, the Swedish diplomat who saved thousands of Hungarian Jews from being killed by the Nazis, has been formally declared dead, 71 years after he disappeared in Hungary in the closing months of World War II.

It is widely believed that Soviet jailers killed Mr. Wallenberg after he was abducted off the street near Budapest, but his fate has remained a lingering mystery. The official Soviet account, issued in 1957, was that Mr. Wallenberg died of a heart attack at age 34 on July 17, 1947, at the Lubyanka headquarters of the K.G.B. in Moscow. But few took that account seriously.

After the fall of the Soviet Union, a comprehensive Russian-Swedish re-examination of the case, completed in 2000, cited evidence suggesting that Mr. Wallenberg was executed in Lubyanka’s prison, but the report stopped short of reaching a definitive conclusion. In June, newly published diaries of Ivan A. Serov, the original head of the K.G.B., shed new light on the case by stating outright that Mr. Wallenberg was executed.

Mr. Wallenberg’s family said last November that they wanted the Swedish authorities to declare him dead. In a statement, they called the step “a way to deal with the trauma we lived through, to bring one phase to closure and move on.”