Around 880 tonnes of hazardous material remain inside the power station, site of one of history's worst nuclear accidents after a tsunami triggered by a 9.0-magnitude earthquake in 2011.

Preparation work needed to start the retrieval is expected to take "12 to 15" years from now, Tepco official Akira Ono told reporters.

This means the earliest they can embark on the removal is now 2037, according to a Tepco document, after the company previously said they hoped to start in the early 2030s.

Dangerously high radiation levels mean that removing melted fuel and other debris from the plant is seen as the most daunting challenge in the decades-long decommissioning project.

Tiny samples of material have twice been collected under a trial project using special tools, but full-fledged extractions are yet to take place.

The new schedule throws into doubt previously stated goals by Tepco and the government to declare the Fukushima plant defunct by 2051.

But Tepco insisted Tuesday the deadline was achievable despite acknowledging it would be "tough".

"There is no need to abandon the target," Ono said, adding it is the firm's "responsibility" to "figure out how to meet it".

Three of Fukushima's six reactors went into meltdown in 2011 after the huge tsunami swamped the facility.