Carles Puigdemont fled after the Catalonian parliament was dissolved by the Spanish government when it made what they decided was an illegal declaration of independence, following a referendum in the region.
A new election since then has given the separatist parties a slim majority, but a court has ruled that if Mr Puigdemont wants to lead a new government, he must be present in the regional parliament to get the authority.
The constitutional court said Tuesday's planned session will be suspended if Mr Puigdemont tries to be re-elected without being physically present in the chamber.
It also ruled he must seek authorisation to attend the session.
Separatist lawmakers had been deciding whether to let him form a new government without being in the building himself, either being represented by another person or appearing via a video link.