Prosecutors will conduct a careful investigation that I will personally oversee

Judge Marco Antonio Posada

Human rights groups say the decision - the first time a Guatemalan court has agreed to investigate the allegations - reflects a welcome change of attitude among the country's judiciary.

Although there is no guarantee either man will be formally charged, campaigners see the move as a major victory in their fight to bring the perpetrators of the killings to justice.

Genocide policy

The court passed separate rulings on Mr Lucas Garcia, who won a rigged election in 1978, and his successor Mr Rios Montt, who seized power in a coup four years later.

Rios Montt: "Nothing to hide"

Both men have been accused of conducting a policy of genocide against the Mayans, who were believed to be supporting left-wing rebels.

Two years ago, a United Nations truth commission report found that Mr Rios Montt in particular oversaw a scorched earth policy, reducing hundreds of Indian villages to ashes.

About 200,000 Guatemalans died in the civil war, in which the left-wing guerrillas fought state forces. Fighting ended following in peace accords in December 1996

'Nothing to hide'

Mr Rios Montt is currently serving as the leader of Guatemala's Congress and as such he enjoys immunity from prosecution.

A party spokesman said he would not comment on the ruling, but in the past he has insisted that he has nothing to hide.

Mr Lucas Garcia, who lives in Venezuela, is reportedly suffering from Alzheimer's disease and has not made any public statement for several years.

Last week a Guatemalan court sentenced three army officers and a priest to between 20 and 30 years in prison for the murder of the prominent human rights campaigner Bishop Juan Gerardi in 1998.