Speculation of so-called helicopter money refuses to die in Japan, despite repeated denials by Bank of Japan Gov. Haruhiko Kuroda.

From Japan-based economists to global investors including Templeton Emerging Markets Group Executive Chairman Mark Mobius, there is a reluctance to rule out the controversial policy coming as soon as next month amid the monetary authority's struggles to stoke growth and inflation. Kuroda has said at least four times since April that helicopter money is not under consideration, and is prohibited by current law. He repeated over the weekend that there remains "ample space for additional easing" under the existing policy framework.

"It's unthinkable that nothing would happen in September," said Daiju Aoki, an economist at UBS Group AG in Tokyo. "The most likely measure would be pseudo-helicopter money where the BOJ will commit to holding Japanese government bonds for a long time."