In a statement on Tuesday, the High Commissioner expressed her “serious concern” that the Guatemalan Congress had taken steps to approve an amendment to legislation that has been in force since the 1996 peace accords, which was crucial to ending 36 years of violent clashes between Government and mainly rural insurgents.

Ongoing investigations into abuse will also be halted, Ms Bachelet said. “This in effect means complete impunity for all those involved in some truly horrendous violations, including crimes against humanity,” she said, warning that such a move risked reopening “old wounds" and could "destroy victims’ trust in the State and its institutions”.If adopted, the amendment will see dozens of people in jail for enforced disappearances, summary executions, sexual violence and torture, freed within 24 hours. Before the amendment can be adopted, Congress must approve it following three separate readings.

Some 200,000 people are estimated to have died during Guatemala’s civil war, according to the UN human rights office (OHCHR). In November, it welcomed the conviction of a former Guatemalan soldier involved in the infamous Dos Erres massacre of indigenous Maya villagers.

OHCHR said that the ruling against Santos López Alonso was “an important step” for transitional justice in Guatemala, although he was one of only six military personnel to have been convicted, amid the frequent use of injunctions to stall the trials of high-level officials.

“I am also deeply worried that if this amendment is approved, it may lead to retaliation against all those courageous victims, witnesses, judges, public prosecutors, lawyers and organizations who have been promoting justice for past crimes in Guatemala,” Bachelet said.

The UN rights chief highlighted that international standards establish limits regarding the adoption of amnesties for the most serious crimes, and pointed out that they are “incompatible with State obligations to prosecute grave violations of human rights.”