Tokyo Correspondent Juliet Hindell: He did not take part in the underground gas attack

It is the first death sentence to be handed down on a member of the cult, which released Sarin gas in Tokyo's underground railway in March 1995, killing 12 people and injuring thousands.

"The accused's criminal responsibility is too heavy to give leniency," said judge Megumi Yamamuro.

Kazuaki Okazaki, who is 37, was found guilty of conspiring with five other sect members in the murder of anti-Aum lawyer Tsutsumi Sakamoto, his wife Satoko and their baby son Tatsuhiko in November 1989.

Prosecution witnesses said they crept into the Sakamoto family home while they slept and injected them with lethal doses of potassium chloride and strangled them.

Okazaki fled the cult the following year, taking about $2m with him.

He was also convicted of killing 21-year-old Shuji Taguchi, who tried to leave the cult in February 1989.

Comparison inevitable

Sect leader Shoko Asahara, 43, has also been charged with murdering the Sakamotos. He is on trial for the subway attack.

Kazuaki Okazaki will not be told the date of his exectuion

BBC Tokyo Correspondent Juliet Hindell said the sentence is bound to be compared with the life sentence handed down to a leading member of the cult dubbed "Dr Death" for his part in the attack on the Tokyo underground railway.

Ikuo Hayashi's apparent remorse and his co-operation in the investigation were believed to have influenced the decision to punish him with life imprisonment instead of the death penalty.

The subway attack - the worst act of terrorism in modern-day Japan - shocked a nation that had taken safety on the streets for granted.

The cult has also been implicated in another nerve-gas attack in central Japan in 1994 that killed seven people; in the deaths of several wayward followers; the near-fatal shooting of Japan's top police official; and the kidnapping and killing of an anti-cult lawyer and his family.

More than 400 of the cult's 10,000 members in Japan were arrested and it was forced to disband. More than 100 members are facing charges connected to the gas attack.