Catalan President Carles Puigdemont is due to bring the results of the region's independence referendum before the regional parliament on Tuesday.
The regional government published the final results on Friday, with 90.18 percent of those taking part voting "yes" on a low 43 percent turnout.But the vote was declared illegal by the Spanish government and obstructed by police.
Many opponents boycotted it and there was a large demonstration against independence in Barcelona on Sunday.
“If there were to be a declaration of independence, it would be unilateral and it would not be recognised,” Loiseau told CNews television on Monday.
“Catalonia cannot be defined by the vote organised by the independence movement just over a week ago,” she went on. “This crisis needs to be resolved through dialogue at all levels of Spanish politics.”
She also repeated the European Union's warning that a breakaway Catalonia would have reapply for membership of the bloc, saying it would have “automatically left the European Union".
It was not clear on Monday whether Puigdemont would declared independence or simply recognise the referendum result.
The Catalan independence campaign has worried some politicians in France, which has its own Catalan-speaking region, known as Pyrénées-Orientales and has also faced a separatist movement in Brittany.