Concern is growing that the Fukushima Daiichi nuclear power plant in Japan is no longer stable after temperature readings suggested one of its damaged reactors was reheating.

The plant's operator, Tokyo Electric Power (Tepco), said the temperature inside No 2 reactor – one of three that suffered meltdown after last year's earthquake and tsunami – may have reached 82C on Sunday.

Tepco said there was no evidence that the melted fuel inside had reached criticality. The utility reportedly increased the amount of cooling water being injected into the reactor along with a boric acid solution, which is used to prevent the fuel from undergoing sustained nuclear reactions.

Confirmation that the temperature has risen above 80C could force the government to reverse its declaration two months ago that the crippled plant was in a safe state known as cold shutdown.

Cold shutdown is achieved when the temperature inside the reactors remains below 100C and there is a significant reduction in radiation leaks. Given that Tepco assumes a margin of error of 20C, the actual temperature could have risen to 102C.

Plant workers are unable to take accurate readings of the temperature inside the damaged reactor because radiation levels are still too high for them to enter and examine the state of the melted fuel, which is thought to be resting at the bottom of the reactor's pressure vessel.

The result has been a series of wildly different readings: two other thermometers positioned at the bottom of No 2 reactor showed the temperature at 35C, local media reported.

Tepco said it did not know the cause of the apparent temperature rise, but speculated that it might be due to problems with the supply of coolant or a faulty thermometer.

"We believe the state of cold shutdown is being maintained," said Junichi Matsumoto, a company spokesman. "Rather than the actual temperature rising, we believe there is high possibility that the thermometer concerned is displaying erroneous data."

Tepco was forced to inject additional cooling water into the same reactor last week after the temperature started rising at the beginning of the month.