The letter was sent shortly before the 10 a.m. Thursday deadline that Mr. Rajoy had set for Mr. Puigdemont to clarify whether Catalonia had declared independence.
The question arose after Mr. Puigdemont delivered a perplexing speech before Catalonia’s regional Parliament on Oct. 10, in which he appeared to declare independence but then immediately suspended the decision. Separatist lawmakers then signed a declaration of independence, but without first voting on the text, as had been expected.
In recent days, government officials in Madrid have repeatedly warned that Mr. Rajoy would consider anything short of a clear withdrawal of the declaration of independence to be unacceptable blackmail, after what he deemed an unsatisfactory response from Mr. Puigdemont on Monday.
Mr. Rajoy initiated a request last week to invoke Article 155 of the Spanish Constitution — a broad, forceful tool that has never been used — that would allow him to take control of Catalonia.
The article would give Madrid the authority to suspend Mr. Puigdemont and other Catalan lawmakers, and to take charge of the region’s autonomous administration, although Mr. Rajoy has not publicly committed to an emergency intervention.
It is unclear what Mr. Rajoy will propose to his cabinet on Saturday, but he may try to gradually raise pressure on the fragile coalition of Catalan separatists, rather than risk a forceful intervention that could backfire and galvanize the independence movement. Politicians in Madrid have also recently demanded that Catalonia hold regional elections as soon as possible, but Mr. Puigdemont made no mention of such a vote in his letter on Thursday.
The separatist leaders of Catalonia are already claiming that Madrid has used disproportionate means to push them out of office, with the help of Spanish police officers and the courts. On Tuesday evening, about 200,000 demonstrators gathered in central Barcelona, according to the local police, to demand the release of two separatist leaders who were sent to prison without bail, pending a trial on sedition charges. In his letter on Thursday, Mr. Puigdemont mentioned the arrest of the two leaders as evidence of Spain’s repressive stance.
Mr. Puigdemont is committed to honoring the result of a highly divisive referendum on independence, which went ahead on Oct. 1 despite having been declared illegal by the Constitutional Court and despite a violent crackdown by the Spanish police. Almost 2.3 million people cast ballots, and 90 percent voted for independence, according to the Catalan government.
But the threat of declaring independence then prompted hundreds of companies to relocate their headquarters outside Catalonia, further straining the unwieldy separatist coalition that holds a majority of the seats in the regional Parliament. Hard-line secessionists want an abrupt and unilateral rupture with the central government in Madrid, while conservative and more moderate separatists are increasingly worried about the economic consequences for Catalonia if companies leave the region.
Mr. Puigdemont sent his latest letter after an emergency meeting of his conservative party late Wednesday, during which party lawmakers gave clear support for not withdrawing the declaration of independence, according to local news reports.
Using constitutional powers, Mr. Rajoy could appoint a caretaker administration in Catalonia. Mr. Puigdemont, on the other hand, could face sedition charges and ultimately a heavy prison sentence for presenting a unilateral declaration of independence that violates Spain’s Constitution.