TOKYO:A Tokyo-based art collective has admitted doctoring a mural by the late master Taro Okamoto about the horrors of a nuclear explosion with an image of the crippled Fukushima plant, reports said Thursday.

ChimPom, a group of six avant-garde artists, said they came up with the clandestine image which caused a stir when it was added on April 30 to “Myth of Tomorrow” on display at a busy Tokyo train station, the Tokyo Shimbun newspaper reported.

Police are investigating whether an offence has been committed, Japan's Jiji Press agency said.

The artists sparked controversy in 2008 when they depicted an airplane vapour trail forming Japanese characters meaning “atomic bomb in Hiroshima”, the first city to be destroyed by a nuclear weapon when the United States dropped an atomic bomb on it on August 6, 1945.

ChimPom could not be immediately reached for confirmation.

A small wooden panel - which shows black smoke billowing from reactor buildings resembling those at Fukushima and mimics Okamoto's painting style - was attached to the wall without causing damage to the original 30-metre (100-foot) long wall painting.

Okamoto, who was born 100 years ago and died in 1996, is one of Japan's best-known modern artists. Strongly influenced by Pablo Picasso, he is known for his abstract paintings and sculptures, including his “Tower of the Sun”erected for the Osaka Expo in 1970.

“Myth of Tomorrow”, created in Mexico in 1968-69, went missing for years but was rediscovered in 2003, returned to Japan and finally installed at a pedestrian overpass at the capital's Shibuya railway station in 2008.

Japan's massive earthquake and tsunami on March 11 destroyed the cooling systems of the Fukushima plant, causing explosions and fires. The plant has since leaked radioactive substances into the air, ground and sea.