Fonseka, in a statement released through his democratic National Alliance (DNA) party, said he did not accept Friday's verdict in a court martial which held he was guilty of corruption in defence procurement deals.
DNA spokesman Anura Kumara Dissanayake said the party rejected the court martial process as seriously flawed and would make a formal appeal to the civilian Court of Appeal "as a matter of routine".
"There is no justice in the country and we can't have any faith in the judiciary, but we will make a formal appeal as a matter of routine," Dissanayake told reporters in Colombo.
The military tribunal recommended that Fonseka be jailed for three years, a decision that is widely expected to be ratified by President Mahinda Rajapakse on his return from New York later this month.
Fonseka, 59, who became a national hero after leading the military to victory over ethnic Tamil separatists in May last year, fell out with Rajapakse and unsuccessfully tried to unseat him at January polls.