Chubu Electric Power's Hamaoka plant -- located in an area of central Japan at risk of a potential "megaquake" -- is undergoing regulatory safety checks, with the aim of restarting two reactors.

But the company's president said late Monday that the estimated maximum seismic ground motion that the plant could experience during a quake "may have been underestimated".

"This incident could seriously affect the (safety) review process and undermine the trust of local communities and other stakeholders in our nuclear business, and potentially shake its very foundation," he told reporters during an emergency press conference.

Japan pulled the plug on nuclear power after a colossal earthquake and tsunami sent three reactors at the Fukushima atomic plant into meltdown in 2011.

However, the resource-poor nation now wants to revive atomic energy to reduce its heavy dependence on fossil fuels, achieve carbon neutrality by 2050 and meet growing energy needs from artificial intelligence.

The world's biggest nuclear plant, Kashiwazaki-Kariwa, is expected to restart operations later this month, pending final approval from the Nuclear Regulation Authority (NRA).

The estimate of maximum seismic ground motion is important for the earthquake-resistant design of nuclear plants.

In September 2023 the NRA approved Chubu Electric's estimate of 1,200 gal, a unit of acceleration used to measure the intensity of quakes.

But in February last year the NRA received information from a whistleblower that the utility "may have used data different from what was presented" to the nuclear watchdog, NRA official Keiichi Watanabe told AFP on Tuesday.

Since then, the watchdog has been conducting its own probe, he said.

The NRA suspended its safety review process for the Hamaoka plant in late December, and the issue "will be discussed in meetings open to the public", the first of which will be held on Wednesday, he said.

The Hamaoka plant is located in Omaezaki, Shizuoka prefecture, near a seismic fault line where a massive Pacific earthquake is expected to occur in the coming years or decades.

In 2024, the Japan Meteorological Agency issued its first special advisory for a possible "megaquake" in the area, known as the Nankai Trough, before lifting it a week later.

The government has said a quake in the 800-kilometre undersea trench and subsequent tsunami could kill as many as 298,000 people and cause up to $2 trillion in damages.

Chubu Electric said it had set up a panel of lawyers to look into the matter.