It is the first time Japanese authorities have admitted the possibility that the fuel suffered “melt-through” – a more serious scenario than a core meltdown.

The report, which is to be submitted to the International Atomic Energy Agency (IAEA), said fuel rods in reactors No 1, 2 and 3 had probably not only melted, but also breached their inner containment vessels and accumulated in the outer steel containment vessels.

The plant’s operator, Tokyo Electric Power (Tepco), says it believes the molten fuel is being cooled by water that has built up in the bottom of the three reactor buildings.