A Nigerian court Monday sentenced a former aide to late dictator Sani Abacha to death by hanging over the 1996 murder of the wife of a presidential candidate after a long-delayed trial.

Judge Mojisola Dada of the Lagos high court ruled that Army Major Hamza Al-Mustapha, chief security aide to Abacha, was guilty of conspiracy and murder of Kudiratu Abiola, wife of late presidential candidate Moshood Abiola.

An aide to Abiola, Lateef Sofolahan, was found guilty of the same charges and also sentenced to death by hanging after the trial which had been repeatedly delayed since 1999.

Kudiratu Abiola was shot dead on 4 June 1996 in the economic capital Lagos. Her husband was the presumed winner of 1993 presidential elections, which were later annuled, prompting unprecedented protests in Nigeria.

"Evidence was manifestedly heavy that they killed Kudiratu Abiola. In view of this, they are guilty of conspiracy and murder," Dada said in her ruling which lasted several hours.

"The prosecution has proved its case beyond all reasonable doubt. In view of this, they should be hanged."

The annulment of the 1993 elections helped paved the way for Abacha's rise to power.

Al-Mustapha was the powerful chief security officer to Abacha, who ruled Nigeria with iron hand from November 1993 to July 1998 when he died.

Abiola was a wealthy businessman and presidential candidate jailed in 1994 after he challenged the military's decision to annul the vote. He died in jail a month after Abacha's death in circumstances yet to be clarified.

Abacha had allegedly set up a so-called "strike force" which hunted down opponents of the regime, a number of whom were either shot dead or jailed, while some fled the country.

Nobel literature laureate Wole Soyinka, a harsh critic of Abacha, was among those who went into exile.

The judge said Sofolahan "acted as Judas Iscariot. He was friend to the Abiola family in the open and enemy in secret. He sacrificed his master (Abiola) because of his personal greed. He was a viper."

Al-Mustapha's defence lawyer Olalekan Ojo said he would appeal the court judgement.

Nigeria has not officially carried out a death penalty in some 15 years, with earlier executions having prompted harsh criticism from rights groups. Nigeria's security forces have however been repeatedly accused of summary executions.