Japan's prime minister at the time of the 2011 earthquake and tsunami has revealed that the country came within a "paper-thin margin" of a nuclear disaster requiring the evacuation of 50 million people.

In an interview with The Telegraph to mark the fifth anniversary of the tragedy, Naoto Kan described the panic and disarray at the highest levels of the Japanese government as it fought to control multiple meltdowns at the crippled Fukushima Daiichi nuclear power station.

He said he considered evacuating the capital, Tokyo, along with all other areas within 160 miles of the plant, and declaring martial law. "The future existence of Japan as a whole was at stake," he said. "Something on that scale, an evacuation of 50 million, it would have been like a losing a huge war."

Mr Kan admitted he was frightened and said he got "no clear information" out of Tepco, the plant's operator. He was "very shocked" by the performance of Nobuaki Terasaka, his own government's key nuclear safety adviser. "We questioned him and he was unable to give clear responses," he said.

"We asked him – do you know anything about nuclear issues? And he said no, I majored in economics."