The Singapore Prison Service (SPS) will investigate a number of articles published by the Malaysian media that were purportedly penned by a Malaysian drug mule on death row here, after the inmate in question, Pannir Selvam Pranthaman, told SPS that he did not write them.

In a statement last night, SPS said that the articles had been written in Pannir's name by someone else.

"This appears to be part of an orchestrated campaign to put pressure on the Singapore Government," SPS said.

"SPS will investigate further the circumstances relating to this matter," it added.

Pannir is fighting the rejection of his clemency plea by President Halimah Yacob.

Malaysian news portal Malay Mail on Monday published a first-person account of life in prison said to be written by Pannir.

It claimed that he was subjected to "psychological torture", locked up in a very hot room with the lights on for two weeks.

In the article, the author described being denied the right to be baptised, and said the treatment in prison reminded him of stories that he had read of Nazi concentration camps.

The Malay Mail published another letter purportedly written by Pannir on Aug 15, detailing his thoughts and goals while in prison.

Pannir was convicted of importing 51.84g of heroin in 2017, and was due to be hanged in May this year.

His clemency petition to President Halimah was rejected in the same month, but he received a stay of execution after he declared his intentions to mount a legal challenge against the decision.

Subsequently, Law and Home Affairs Minister K. Shanmugam revealed that his Malaysian counterpart had written to the Singapore Government over the case.

Mr Shanmugam said that it was one of three requests made by the Pakatan Harapan government to stop the execution of Malaysians in Singapore since it took power after the election in May last year.

But Singapore cannot make exceptions for Malaysians who have been sentenced to death as it would undermine the rule of law here, Mr Shanmugam explained.

The Singapore Government will not intervene when the courts have already imposed a sentence and there are no legal reasons to do so, he added.

"It is not tenable to give a special moratorium to Malaysians, and impose it on everyone else, including Singaporeans who commit offences which carry the death penalty," he said.

Pannir has applied to the High Court for permission to start judicial review proceedings.