Japan’s former prime minister Junichiro Koizumi has labelled the country’s current leader, Shinzo Abe, a “liar” for telling the international community that the situation at the wrecked Fukushima Daiichi nuclear power plant is under control.

Koizumi, who became one of Japan’s most popular postwar leaders during his 2001-06 premiership, has used his retirement from frontline politics to become a leading campaigner against nuclear restarts in Japan in defiance of Abe, a fellow conservative Liberal Democratic party (LDP) politician who was once regarded as his natural successor.

Abe told members of the International Olympic Committee (IOC) in Buenos Aires in September 2013 that the situation at Fukushima Daiichi nuclear power plant was “under control”, shortly before Tokyo was awarded the 2020 Games.

IOC officials were concerned by reports about the huge build-up of contaminated water at the Fukushima site, more than two years after the disaster forced the evacuation of tens of thousands of residents.

“When [Abe] said the situation was under control, he was lying,” Koizumi told reporters in Tokyo. “It is not under control,” he added, noting the problems the plant’s operator, Tokyo Electric Power (Tepco), has experienced with a costly subterranean ice wall that is supposed to prevent groundwater from flowing into the basements of the damaged reactors, where it becomes highly contaminated.

“They keep saying they can do it, but they can’t,” Koizumi said. He went on to claim that Abe had been fooled by industry experts who claim that nuclear is the safest, cleanest and cheapest form of energy for resource-poor Japan.

“He believes what he’s being told by nuclear experts,” Koizumi said. “I believed them, too, when I was prime minister. I think Abe understands the arguments on both sides of the debate, but he has chosen to believe the pro-nuclear lobby.”

After the Fukushima crisis, Koizumi said he had “studied the process, reality and history of the introduction of nuclear power, and became ashamed of myself for believing such lies”.

Abe has pushed for the restart of Japan’s nuclear reactors, while the government says it wants nuclear to account for a fifth of Japan’s total energy mix by 2030. Just three of the country’s dozens of nuclear reactors are in operation, and two will be taken offline later this year for maintenance.

Koizumi, 74, has also thrown his support behind hundreds of US sailors and marines who claim they developed leukaemia and other serious health problems after being exposed to Fukushima radiation plumes while helping with relief operations – nicknamed Operation Tomodachi (friend) – following the 11 March 2011 earthquake and tsunami.

In 2012 the service personnel launched a lawsuit accusing Tepco of failing to prevent the accident and of lying about the levels of radiation from the stricken reactors, putting US personnel at risk.

Most of the 400 plaintiffs were aboard the USS Ronald Reagan, a nuclear-powered aircraft carrier that was anchored off Japan’s north-east coast while helicopters flew emergency supplies to survivors of the tsunami, which killed almost 19,000 people.

Medical experts, however, said the sailors would have received only small, non-harmful doses of radiation; a US defence department report published in 2014 said no link had been established between the sailors’ health problems and their exposure to low doses of Fukushima radiation.

Koizumi, who met several of the sick servicemen in San Diego in May, plans to raise $1m by the end of next March to help cover the sailors’ medical expenses.

“I felt I had to do something to help those who worked so hard for Japan,” he said. “That won’t be enough money, but at least it will show that Japan is grateful for what they did for us.”

Despite his opposition to Abe’s pro-nuclear policies, Koizumi was complimentary about his performance as prime minister during his second time in office in the past decade.

“As far as nuclear power is concerned, we are totally at odds,” Koizumi said. “But I think he’s reflected on the mistakes he made during his first time as leader and is doing a much better job second time around.”

In political longevity terms, Abe’s performance could hardly be worse. He resigned in September 2007 after less than a year in office, following a series of ministerial scandals, a debilitating bowel condition and a disastrous performance by the LDP in upper house elections.