Anton Balasingham, the chief negotiator and ideologue of the Liberation Tigers of Tamil Eelam, died at his home in London on Thursday.
He was 68.
In October, Balasingham was diagnosed with bile duct cancer. He was also suffering from diabetes and had kidney problems following a transplant operation several years ago.
The funeral will be in London, a family member said. He leaves his Australian-born wife Adel who is also a key figure in the LTTE.
Both have written several books on Sri Lanka's Tamil separatist movement.
Balasingham was seen as a moderate who was keen to cut a political deal and end decades of ethnic bloodshed.
He did not attend the last round of talks in Geneva in October because of failing health and his last outing was in February in Celigny, near Geneva when the Tigers and the government met face to face.
Balasingham was staying in London after leaving Sri Lanka.
Before leaving for London he worked for a Tamil newspaper and had a stint with the British High Commission Colombo as a translator.
He ended his self-imposed exile and returned home in April 2002 to attend a landmark press conference by LTTE supremo Velupillai Prabhakaran before opening Norwegian-brokered peace talks with Colombo.
However, he did not stay in Sri Lanka and returned to London from where he traveled to Japan, Thailand, Norway, Germany and Switzerland to attend more than half a dozen rounds of peace talks.
From the Thimpu talks in 1985 to the first round of the Geneva peace talks with the government held in February 2006, Balasingham was the head of the LTTE delegation.
Shortly after doctors gave him a few weeks to live, he said he was sad that his illness prevented him from contributing to the cause of minority Tamils at home.
'I am deeply sad that I am crippled by this illness, unable to contribute anything substantial towards the alleviation of the immense suffering and oppression of my people,' pro-rebel website Tamilnet.com quoted him as saying.
Balasingham was the main contact for peace broker Norway and other key international players involved in attempts to bring about a peaceful resolution to the island's drawn-out Tamil separatist conflict.