From 1721 this was the Whig politician Robert Walpole, who held office for twenty-one years. Walpole chaired cabinet meetings, appointed all the other ministers, dispensed the royal patronage and packed the House of Commons with his supporters. Under Walpole, the doctrine of cabinet solidarity developed. Walpole required that no minister other than himself have private dealings with the king, and also that when the cabinet had agreed on a policy, all ministers must defend it in public, or resign. As a later prime minister, Lord Melbourne, said, "It matters not what we say, gentlemen, so long as we all say the same thing."
Is there an answer to this question (If it cannot be answered, say "unanswerable"): Who was the prime minister in the 17th century?
unanswerable