Rajoy called an emergency meeting after Catalonia's President Carles Puigdemont announced Tuesday that he had accepted the mandate for "Catalonia to become an independent state" following a banned referendum earlier this month.
This formal requirement is needed to trigger the article 155 though the Constitution does not establish any specific time frame for the answer.
Addressing the Catalan Parliament in Barcelona on Tuesday evening, Catalan leader Carles Puigdemont said: "We call on global states and organisations to recognise the Catalan republic as an independent and sovereign state".
Spain's main opposition party, the Socialist party, agreed with the ruling People's Party that the Catalan regional government has to clarify whether there was a declaration of independence or not, the Socialist's leader Pedro Sanchez said on Wednesday.
Failing that, Madrid will invoke Article 155 of the constitution allowing it to suspend the region's autonomy and impose direct rule.
Mr Rajoy issued the demand following a special Cabinet meeting to respond to an announcement from Mr Puigdemont, that he was proceeding with a declaration of independence but was suspending it for several weeks to facilitate negotiations.
Spain and Catalonia now enter into the unknown, as Madrid has repeatedly said independence is not up for discussion.
"Yesterday I tried to send a message of calmness and to remind people that we are facing a political problem that we need to solve with politics and not with police", he said.Hundreds of people were injured in Catalonia on October 1 as Spanish national police sought to prevent the referendum going ahead. And French President Emmanuel Macron rejected Puigdemont's call for European Union mediation, saying he was confident Madrid could handle the situation.
He accused Mr Puigdemont of having created "deliberate confusion" and said he wanted to restore "certainty".
The poll was marred by violence after Spanish police acting on court orders attempted to stop the vote by raiding polling stations, seizing ballot boxes, beating voters and firing rubber bullets at crowds.
But in discussing the controversy now raging over Catalonian independence, Press Secretary Sarah Sanders also made clear Tuesday that the administration stood by the president's remarks September 26 that "Spain is a great country and it should be united".