The Court of Appeals in Tunis on Tuesday ruled that two civilians could be tried in military courts.
Lawmakers Saif Eddine Makhlouf and Nidal Saudi had appealed a decision that they be tried before military courts. However the judges ruled that military courts have jurisdiction to try civilians.
In September, a military judge jailed Saudi and Makhlouf, of the Karama Party, bringing the total number of imprisoned MPs to five. Makhlouf is a frequent critic of President Kais Saied.
The court of appeals' decision has sparked widespread criticism and rejection from lawyers, politicians and human rights defenders who said the ruling marks a "black day in the history of the judiciary".
Earlier on Sunday Saied met with the highest judicial bodies in the country, sparking widespread controversy as a public attempt by the president to interfere and control the judiciary.
Saied has held nearly total power since 25 July when he sacked the prime minister, suspended parliament and assumed executive authority citing a national emergency.
He appointed a prime minister on 29 September and a government has since been formed.
The majority of the country's political parties slammed the move as a "coup against the constitution" and the achievements of the 2011 revolution. Critics say Saied's decisions have strengthened the powers of the presidency at the expense of parliament and the government, and that he aims to transform the country's government into a presidential system.
On more than one occasion, Saied, who began a five-year presidential term in 2019, said that his exceptional decisions are not a coup, but rather measures within the framework of the constitution to protect the state from "imminent danger".