Catalonia's government has insisted it will not meet Madrid's demands in the wake of a disputed vote on secession.
There have been protests over the detention of two leaders of the separatist movement.
Jordi Sánchez and Jordi Cuixart are being held while they are investigated for sedition - a move which many on the pro-independence side see as politically motivated.
The men were leading figures in the 1 October independence vote, which Madrid branded illegal.
After the referendum, Catalan leader Carles Puigdemont signed a declaration of independence but halted its implementation and called for negotiations.
Prime Minister Rajoy has given Mr Puigdemont until Thursday to come up with a definitive answer on the independence question or face the implementation of direct rule under Article 155.
"All I ask of Mr Puigdemont is that he acts sensibly... to put the interest of all citizens first," Mr Rajoy told the Spanish parliament on Wednesday.
But if the separatist leader does not provide a satisfactory response by 10:00 local time (08:00 GMT) on Thursday, he "will provoke the application of Article 155", Ms Saenz de Santamaria said.
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