The recommendation drew an angry response from the region's leader, who also dismissed moves by the State Attorney - a separate, government-appointed body prosecuting in the case - to push for the lesser charge of sedition.
Madrid imposed direct rule on the wealthy northeastern region in October 2017 after it declared its independence on the basis of a referendum that Spanish courts ruled was unconstitutional.
The nine Catalan politicians were jailed without bail for their role in that secession drive, which laid bare deep divisions between the region's pro- and anti-Spain camps, and remain in custody pending their Supreme Court trial.
The prosecutor, which also filed a charge of misuse of public funds against the nine, sought the longest term of 25 years for Oriol Junqueras, the leader of pro-Catalan-independence party ERC.
The State Attorney, which is part of the justice ministry, said it was seeking maximum sentences of 12 years.
Reiterating calls for all charges to be dropped, Catalonia's pro-secession government head Quim Torra said the pursuit of the campaigners through the courts showed "absolute disdain for imprisoned democrats".
"Today, the government has missed a golden opportunity to take this conflict out of the courts and return it to a political matter," he said.
The prosecutor has precedence in the trial, expected to begin in January.
The Attorney's bid for the lesser charge of sedition coincides with efforts by the Socialist government of Pedro Sanchez to reduce tensions between Madrid and Barcelona. Justice Minister Dolores Delgado said it was acting purely on judicial criteria.
While refusing to countenance any move towards Catalan independence, Sanchez has adopted a more conciliatory approach towards the region than his predecessor Mariano Rajoy.
With the Socialists holding only 84 of 350 parliamentary seats, he is also relying on the support of Catalan lawmakers to get his 2019 budget passed through the house.
The ERC's Junqueras was deputy to the then Catalan leader Carles Puigdemont during the vote for independence last year. Puigdemont fled the country after Madrid took control of the regional government - which it handed back in December - and has been in self-imposed exile since.
The prosecutor recommended sentences of up to 17 years on rebellion and misuse of funds charges for the other eight jailed Catalan politicians, and is seeking lesser charges for nine others accused of involvement in the campaign who are not in custody.
The leaders' defence team must present its case in the next few weeks and the trial is expected to begin in January.
The prosecutor is also seeking up to 11 years in prison for four others, including the head of the Catalan police at the time of the independence declaration, in a separate High Court case. (Additional reporting by Jesus Aguado; Editing by John Stonestreet)
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