PKR vice-president Shamsul Iskandar Md Akin confirmed receiving a letter from Anwar Ibrahim warning him about Dr Mahathir Mohamad.

“Correct,” he replied in a text message to Malaysiakini when questioned if the news report regarding the letter was true.

However, the Bukit Katil MP declined to comment further.

Meanwhile, PKR president Dr Wan Azizah Wan Ismail appeared clueless about the letter when quizzed about it.

"I'm sorry I will have to check with Shamsul," she said when met in Sungai Besar.

The opposition leader said her husband’s lawyers or Shamsul himself would know better if there was a letter.

The Star yesterday cited a source as saying Anwar had issued a second letter from behind bars.

This time the letter was reportedly addressed directly to Shamsul, advising him to exercise wisdom and restraint when dealing with Mahathir.

Previously, Anwar in his first letter had warned all PKR leaders to be cautious of working with Mahathir.

The PKR de facto leader had lamented that some party leaders were in cahoots with Mahathir and former finance minister Daim Zainuddin who were elitists and staunch defenders of cronyism.

He warned that working with them could derail PKR's struggles.

He also lamented that while Azmin, PKR vice-president Tian Chua, and PKR secretary-general Rafizi Ramli were authorised to negotiate on the Citizens' Declaration, it had turned out to be a flawed Mahathir document.

The Citizens’ Declaration aims to remove Prime Minister Najib Abdul Razak from power.

Anwar is currently serving a five-year prison sentence for a sodomy conviction, which he claimed was based on a fabricated charge by his political rivals.

The authorities, however, have denied this.