The Tokyo Electric Power Co (TEPCO) had been scheduled to start removing melted fuel from deep inside one of the mangled reactors next year, a decade after the nation's worst ever nuclear crisis was triggered by a tsunami.

The process is considered the most difficult of the massive decommissioning programme, which is expected to take three to four decades to complete.

TEPCO had planned to develop a robot arm in Britain that would have arrived in Japan next year to start work -- but chief decommissioning officer Akira Ono told a news conference that a recent spike in Covid-19 infections in the UK had delayed this.

"It will now be difficult to transfer the system in January as scheduled," Ono said, adding he hoped the delay would be limited to a year.

The removal process is expected to take several years for the number two unit, which is estimated to contain some 237 tonnes of debris, Kyodo News said.

Altogether, three melted-down units are estimated to house around 880 tonnes of debris.

Ono said the delay was "regrettable" but insisted that it was unlikely to affect the entire plan to decommission the plant between 2041 and 2051.

The company also faces other difficult challenges, including working out how to dispose of large quantities of contaminated water stored in containers at the plant site.

In the worst nuclear disaster since Chernobyl in 1986, reactors one, two and three at the Fukushima Daiichi power plant melted down after a deadly earthquake and tsunami struck Japan in 2011.

The tsunami killed around 18,000 people and wreaked widespread devastation, and the nuclear meltdown forced the evacuation of areas near the plant.

An evacuation order has been partially lifted but regions affected by the disaster have struggled to attract residents back, with many still concerned about radiation despite government assurances.