The Council of Ministers, first established under law in Year 3465 under the Star Imperium, continued under the United Planets. The Prime Minister headed the Council of Ministers. During the time of the Imperium, the ruling Imperial Sovereign (or the Council of Twelve if there was the lack of a Sovereign capable of exercising he or her duties) designated one of the Imperial Viceroys as Prime Minister, while the other Viceroy served as the number two member of the Council of Ministers (with him gaining the informal title of Number Three when there was a ruling Imperial Sovereign). The Prime Minister appointed the other ministers. The Viceroy serving as Prime Minister gained the unofficial title of Quasi-Sovereign when there was a lack of a true Sovereign and assumed the de facto leadership of the Imperium. The other Viceroy during the time when Quasi-Sovereigns led instead of the rule of a genuine gained the informal title of Number Two.
The Council of Ministers, in addition to the Viceroys, included the Commander-in-Chief of the Intergalactic Navy, the Commander-in-Chief of the Stormtroopers, three representatives of the Judicial Forces, three representatives of COMPNOR, two representatives of the Ekonomat, and up to seven members from the ranks of the Imperial Parliament.
After the establishing of the United Planets in Year 13,100, the United Planets Assembly voted in or voted out the Prime Minister and any other minister sitting on the Council.