The mastermind behind the 1995 Tokyo subway gas attacks faces execution after Japan's supreme court rejected his last legal appeal today.

Shoko Asahara, who led the Aum Shinrikyo cult and believed himself to be a reincarnation of the Hindu god Shiva who would bring about the apocalypse by means of the sarin nerve gas attack, could be executed within six months.

He was found guilty in 2004 of orchestrating a string of attacks that killed 27 people, including the 12 who died in the Tokyo subway attacks when Aum members boarded busy commuter trains and released bags of sarin among the passengers.

Japan is the only developed country other than America to execute people, and the impact of the 1995 Aum Shinrikyo attacks is considered to be a factor in the recent revival of the practice after an unofficial moratorium in the early 1990s.

A justice ministry official, speaking on condition of anonymity because of ministry rules, said Asahara's defence team could still forestall his execution by applying for a retrial or an emergency appeal.

If the defence team does nothing, Asahara will be executed within six months of receiving official notice from the court of today's ruling, the official said. The execution also requires the approval of the justice minister, he said.

Asahara is nearly blind and was born Chizuo Matsumoto before changing his name following a religious conversion while travelling in the Himalayas. He attracted up to 40,000 wealthy and highly-educated Japanese to his cult with claims that only members would be able to escape a coming apocalypse.

The appeal had sought to demonstrate that Asahara was mentally unstable and did not fully understand the proceedings of the court.

The blind former leader, who once commanded the powerful Aum Shinrikyo cult of about 40,000 members, often mumbled incoherently during his eight-year trial, interrupting sessions with bizarre outbursts in gibberish or in broken English. His lawyers say they have never been able to carry out a coherent discussion with their client.

But last month, a court-appointed psychiatrist found Asahara could be feigning mental illness, and said he was competent to stand trial.

In the final appeal, the lawyers claimed both the high court decision to throw out the appeal and the mental evaluation were improper.

In addition to the subway attack, Asahara was also convicted of plotting a 1994 gas attack in the central Japanese city of Matsumoto that killed seven people, the kidnapping and murder of an anti-cult lawyer and his family, and other slayings.

About a dozen other Aum leaders have been sentenced to death, but none have been executed. Most of their cases are still pending appeal in higher courts.

Aum has continued under another name, Aleph. The organisation is under close surveillance and Japanese police suspect the group still follows Asahara's teachings.