TOKYO — In a development that is likely to delay efforts to bring the Fukushima Daiichi Nuclear Power Station under control, the plant’s operator said Thursday that one reactor, No. 1, had sustained much more damage than originally thought and was leaking water.

The company released a plan last month to bring the plant into a relatively stable state in six to nine months, but that was predicated on the notion that it could efficiently cool the fuel in several reactors — a harder task if water is leaking out. The company had long suspected that the containment vessels at two other reactors were breached and leaking, but it had hoped the No. 1 reactor was intact and therefore easiest to bring under control.

The company, Tokyo Electric Power Company, or Tepco, was able to better assess the reactor on Thursday because workers had recently been able to get close enough to fix a water gauge. It showed that the water level in the reactor was much lower than expected despite the infusion of tons of water since a devastating earthquake and tsunami knocked out the plant’s crucial cooling systems.

One of the most startling findings announced Thursday was that water levels in the reactor vessel, which houses the fuel rods, appeared to be about three feet below where the bottom of the fuel rods would normally stand. Ever since the reactor shut down, workers’ primary task has been to keep pouring water into the reactors to ensure the nuclear fuel remained covered so that it would not melt. But the new information suggests the fuel was uncovered for at least some time, probably early in the crisis.