One of the reactors at the crippled Fukushima Daiichi power plant suffered a nuclear meltdown, Japanese officials admitted for the first time today.

Engineers from the Tokyo Electric Power company (Tepco) entered the No.1 reactor for the first time and saw the top five feet or so of the core's 13ft-long fuel rods had been exposed to the air and melted down.

They described a pool of molten fuel at the bottom of the reactor's containment vessel.

The company had previously believed that there was enough water inside to keep the fuel stable.

Now the company is concerned that the molten fuel may have breached the bottom of the containment vessel, escaping out into the outside world, causing a environmental disaster.

We will have to revise our plans, said Junichi Matsumoto, a spokesman for Tepco. We cannot deny the possibility that a hole in the pressure vessel caused water to leak.

The reactor was severely damaged and systems compromised after the March 11 earthquake and tsunami crippled the Fukushima Dai-ichi nuclear plant in northeast Japan.