COLOMBO, Dec 14 (Reuters) - The Tamil Tigers's chief negotiator and ideologue Anton Balasingham, who marshalled the rebels through successive rounds of abortive peace talks with the Sri Lankan state, has died of cancer in London, the rebels said on Thursday. "He is dead," a Tiger official told Reuters by telephone from the rebels' northern stronghold of Kilinochchi. He was 68. Balasingham was a trusted confidant of shadowy rebel leader Velupillai Prabhakaran, wrote his speeches and oversaw rebel negotiations that clinched a 2002 ceasefire. But that truce is now in tatters.