Fonseka, who was arrested by military police at his campaign office on Monday night, has been moved to an undisclosed military establishment ahead of court martial proceedings, the military official said.
"He will face a court martial even though he is not a serving officer," said the official, who declined to be named. "The military law applies up to six months from the date of retirement of any officer."
Fonseka, who lost to President Mahinda Rajapakse in the January 26 election, was arrested hours after he vowed to give evidence at any international probe into alleged war crimes committed by troops in the final stages of fighting against Tamil Tiger rebels last year.
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