More than 40 human rights groups from around the world have penned an open letter to Indonesian President Joko Widodo, pleading for the halting of 10 imminent executions.

The letter, published by the International Federation for Human Rights on Tuesday, “condemn[s] in the strongest possible terms” the planned execution by firing squad of a group of prisoners in Nusakambangan prison, in central Java.

A total of 42 human rights and anti-death penalty groups from countries as far afield as Cameroon, France, Iran, Laos, India, Switzerland, Italy, Vietnam and Nigeria have signed the letter, criticising Indonesia’s execution policy and calling for urgent review of the group scheduled to be killed.

The group includes two Australians, Myuran Sukumaran and Andrew Chan, who have been in Indonesian custody since 2005 after leading the so-called “Bali Nine” drug gang who attempted to smuggle eight kilograms of heroin from Indonesia to Australia.

The pair, sentenced to death in February 2006, have languished on death row ever since, with an exhaustive series of appeals and reviews all ultimately unsuccessful.

Chan and Sukumaran had their clemency appeals recently rejected by Widodo, despite intense lobbying from the Australian government. The affair has strained ties between Indonesia and Australia.

The letter claims the rationale behind executions for drug-related crimes are based on “an outdated and criticized” Indonesian study, saying the impact of drugs on Indonesian society was vastly overstated and that there is no evidence that executing those involved with narcotics has any deterrent effect.

Widodo has stood behind the death sentence for Chan and Sukumaran against mounting international pressure, claiming the lives of 4.5 million Indonesians are “in ruin” because of drugs.

The condemned group – said to also include Brazilian, Filipino, Ghanaian, Nigerian and French citizens – was expected to be executed in coming days. However, in an interview with Al-Jazeera, Widodo said the executions would not take place this week. The execution date is tipped to be revealed on Friday.

“Your decision to authorize more executions in the coming weeks and months has tarnished Indonesia’s international image and risks damaging bilateral relations between Jakarta and capitals of abolitionist countries, which represent 70% of the international community,” the letter states.

“Executions are against Article 28(a) of the Indonesian Constitution, which guarantees everyone’s right to life. They are also in breach of Indonesia’s international legal obligations under Article 6 of the International Covenant on Civil and Political Rights (ICCPR), which recognizes every human being’s inherent right to life.”