The senior negotiator for the Tamil Tigers, Anton Balasingham, has died of cancer in London, the BBC has learned.
A Tamil Tiger spokesman in Sri Lanka said an official announcement would be made shortly.
Last month it was reported that Mr Balasingham, 68, had bile duct cancer which had spread to his liver, lungs, abdomen and bones.
He was the chief ideologue of the rebels in their war against the Sri Lankan military.
BBC Sinhala editor Priyath Liyanage says many analysts believe Mr Balasingham has been a moderating force within the Tamil Tigers (LTTE).
He was 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.
"The LTTE supreme council is in session at present discussing arrangements to honour him," the Tigers' Rasaiah Ilanthirayan told the BBC Sinhala service.
This year has seen a sharp escalation of violence in Sri Lanka.
Mr Balasingham held a British passport and lived in London. The UK classifies the Tigers as a terrorist group.
When news of his illness emerged in November, he said: "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," he told the pro-rebel Tamilnet website.
Ten years ago, he underwent a kidney transplant.