A former communist guerilla fighter in Malaysia who has been living in exile in Thailand is in critical condition, his lawyer said, calling on Kuala Lumpur to let him return home.

Chin Peng, who led a bloody guerrilla campaign after World War II and left Malaysia shortly after the end of the 1948-1960 "Emergency", has been hospitalised in Bangkok, his lawyer Darshan Singh Khaira said on Sunday.

"He has been sick for two months... He's in critical condition," he told AFP.

Singh said several Malaysian family members had visited the 86-year-old former leader of the outlawed Communist Party of Malaya (CPM).

Born Ong Boon Hua in Malaysia's north, Chin Peng was made an Officer of the Order of the British Empire (OBE) and won two medals for helping the British fight the Japanese in Malaya during World War II.

He later led the communist party, backed by China, in a guerrilla campaign against the British colonial and Malaysian governments before and after Malaysian independence in 1957.

In 2009, Malaysia's highest court rejected a bid for him to come back but Singh said Prime Minister Najib Razak's government should now allow his return.

"I hope they let him in now... as a saving grace for Najib," he said. "They should let him in to die among his loved ones."

Singh has insisted Chin Peng has the right to return under a 1989 peace agreement between the CPM and Malaysian government which allowed several high-ranking communist leaders to do so.

But the government has rejected all appeals, fearing Chin Peng's return may open old wounds and anger those whose family members were killed during the emergency.

Last month, a Malaysian court charged an opposition politician for criminal defamation over an alleged pro-communist remark, signaling that tensions over the insurgency still run high.

Opposition leaders and activists slammed the charge as undermining a pledge by Najib to grant greater freedom of expression and overhaul security laws ahead of looming elections.