"The Governor must remain aloof from any disagreement, discord, disharmony, discontent or dissension, within individual political parties," a five-judge Constitution bench said in a hard-hitting order.
"The Governor must keep clear of any political horse-trading, and even unsavoury political manipulations, irrespective of the degree of their ethical repulsiveness. Who should or should not be a leader of a political party, is a political question, to be dealt with and resolved privately by the political party itself. The Governor cannot make such issues a matter of his concern."
This is seen as a damning indictment of the governor, who had been accused by the Congress of tacitly supporting its rebels and also helping the BJP by calling an assembly session in December, a month ahead of schedule.
The Nabam Tuki government was "dismissed" in that session held in a makeshift venue.
The court today said the governor could not support the "protests and assertions of an invalid breakaway group."
The judges also noted that Mr Rajkhowa never called for a floor test or even ask the Chief Minister to prove his majority in the house.
In a preview of today's ruling, the court had been scathing in its questions and observations in a previous hearing this year.
"Arunachal experiment to topple state governments may prove deadly and the court has the power to set the clock back," the court had said during hearings in February.
Congress rebels led by Kalikho Pul took power even as the Supreme Court reserved its judgement.
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