Fonseka was arrested on February 8 after being accused of plotting a coup two weeks after losing the presidential election to Mahinda Rajapaksa, the incumbent.
However, the two separate courts martial he is due to face will not tackle that accusation
If convicted of the charges that he engaged in politics while still head of the army and that he violated military procurement procedures he could reportedly face up to five years in jail.
Damien Kingsbury, a professor of International Relations at Deakin University in Melbourne and a specialist on Sri Lankan politics, told Al Jazeera that "despite Sri Lanka having a democratic system, it's one that is very deeply circumscribed by these types of events".
"This is clearly a political trial, and it's probably just a show trial and it has three purposes.
"One is revenge against Sarath Fonseka for standing against Rajapaksa and for undermining his claim for being the victor against the Tamil Tigers.
"Secondly, it's meant to be a demonstrator to the opposition and thirdly it's meant to cow the oppositon to ensure that those who oppose Rajapaksa do so in a cautious manner."
Fonseka's supporters say the charges are politically motivated and an attempt to prevent him from standing in next month's parliamentary polls.
"This must be important for people in authority but for us this is a joke," Anoma Fonseka, the former army general's wife, said.
"For the entire world, this is a joke. We can't expect any justice from this court martial. They have brought trumped-up charges."
Fonseka's supporters have called for nationwide protests demanding his release.
On Monday, Sri Lanka's former chief justice accused the government of acting unconstitutionally by prosecuting Fonseka behind closed doors using military law, rather than using the normal legal system which allows open hearings.
"The arrest detention of General Sarath Fonseka is contrary to articles 13-1 and 13-2 of the constitution, the code of Criminal Procedure, the International Covenant on Civil and Political Rights and Universal Declaration of Human Rights," Sarath Silva told reporters in Colombo.
"It is also contrary to the Army Act and the Chief of Defence Staff Act."
Officials in Rajapaksa's government have accused Fonseka of plotting a coup and the assassination the president, although those accusations will not form part of the court martial.
Former close allies, Rajapaksa once referred to Fonseka as a "national hero" for his role in leading the Sri Lankan military in its final victory over Tamil Tiger rebels.
The defeat of the Tigers in May last year brought an end to more than two decades of bloody civil war.
But the two men fell out shortly after over who should take credit for the victory.
Fonseka has refused to attend the preliminary hearings for the courts martial and reports say he will also stay away once the case begins on Tuesday.
However officials at his Democratic National Alliance party said that he would be represented by a team of lawyers.