The senior negotiator for the Tamil Tigers, Anton Balasingham, is seriously ill with cancer, a website close to the rebels has reported.
Balasingham (l) has represented the rebels in numerous peace talks
The Tamilnet website said that Mr Balasingham has been diagnosed with bile duct cancer.
It says the cancer is in an advanced state and has spread to his liver, lungs, abdomen and bones.
The 68-year-old is the chief ideologue of the rebels in their war against the Sri Lankan military.
Mr Balasingham holds a British passport and lives in London. The UK classifies the Tigers as a terrorist group.
'Crippled by illness'
"It is an unfortunate personal tragedy. However, when compared to the vast ocean of the collective tragedy faced by my people, my illness is merely a pebble," Mr Balasingham told Tamilnet.
"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."
Mr Balasingham has led negotiations with the government since the latest round of face-to-face talks began in September 2002, but was absent from a meeting in Geneva last month because of failing health.
"He is in an incurable stage with the cancer fast spreading to his liver, lungs and bone-marrow," the Tamil Sudar Oli newspaper said in a report written by its editor who is currently visiting London.
Correspondents say that Mr Balasingham had been the main contact for peace brokers such as Norway as well as other key international players involved in attempts to bring about a peaceful resolution to the island's protracted separatist conflict.
Ten years ago, he underwent a kidney transplant.