Fonseka, 59, has been held at a naval detention centre since his arrest on February 8, two weeks after he was defeated in elections by President Mahinda Rajapakse.
The government has yet to specify the charges Fonseka will face, but Defence Secretary Gotabhaya Rajapakse -- the president's brother -- has said he plotted a military coup while in the army.
As the architect of the victory over the Tamil Tiger rebels last May, Fonseka was hailed as a national hero for crushing their decades-long campaign for an independent Tamil homeland.
However, he later fell out with Rajapakse and mounted a failed bid to unseat the president in elections.