Japan is planning to build what could become the world’s largest floating wind farm – a gigawatt-scale project off the Izu Islands to power the island chain and Tokyo.

But the plan, championed by Governor Yuriko Koike, is facing questions over whether it can be delivered by 2035. There are also doubts about the power generation forecast and whether Japan could tackle the technical demands of such a megaproject.

Koike first announced the wind farm project at a climate change conference in...