Radioactive materials released into the environment by the Fukushima nuclear disaster in Japan have caused mutations in butterflies, a study indicates.

Scientists say they've detected an increase in mutations in leg, antennae and wing shape among butterflies collected following the 2011 Fukushima accident, and that laboratory experiments have confirmed the link with the radioactive release.

Two months after the nuclear power plant accident in March 2011, Japanese researchers collected 144 adult pale grass blue butterflies from 10 locations in Japan, including the Fukushima area.

When the accident occurred, the butterflies would have been overwintering as larvae, the researchers said.

In areas with more radiation in the environment, the butterflies had much smaller wings and irregularly developed eyes, they said, a surprising finding.

"It has been believed that insects are very resistant to radiation," lead researcher Joji Otaki from the University of the Ryukyus, Okinawa, told the BBC.

"In that sense, our results were unexpected."

Previous studies have identified birds and butterflies are important tools to investigate the long-term impacts of radioactive contaminants in the environment.

"This study is important and overwhelming in its implications for both the human and biological communities living in Fukushima," said University of South Carolina biologist Tim Mousseau, who studies the impacts of radiation on animals and plants but was not involved in the study.