Remember my joke about "the cat's on the roof"? If this is what they admit to, it was probably worse.

Fuel rods inside one of the reactors at the Fukushima Daiichi nuclear power plant may have completely melted and bored most of the way through a concrete floor, the reactor's last line of defence before its steel outer casing, the plant's operator said.

Tokyo Electric Power (Tepco) said in a report that fuel inside reactor No 1 appeared to have dropped through its inner pressure vessel and into the outer containment vessel, indicating that the accident was more severe than first thought.

The revelation that the plant may have narrowly averted a disastrous "China syndrome" scenario comes days after reports that the company had dismissed a 2008 warning that the plant was inadequately prepared to resist a tsunami.

Tepco revised its view of the damage inside the No 1 reactor – one of three that suffered meltdown soon after the 11 March disaster – after running a new simulation of the accident.

It would not comment on the exact position of the molten fuel, or on how much of it is exposed to water being pumped in to cool the reactor. More than nine months into the crisis, workers are still unable to gauge the damage directly because of dangerously high levels of radiation inside the reactor building.

"Uncertainty involved in the analysis is significant, due to the uncertain nature of the original conditions and data used," Tepco said in a report. It said the concrete "could have been penetrated", but added that the fuel remained inside the reactor's outer casing.

Previously, the firm had said that only some of the fuel had burned through its inner pressure vessel and dropped into the containment vessel.