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In non-Commonwealth countries the prime minister may be entitled to the style of Excellency like a president. In some Commonwealth countries prime ministers and former prime ministers are styled Right Honourable due to their position, for example in the Prime Minister of Canada. In the United Kingdom the prime minister and former prime ministers may appear to also be styled Right Honourable, however this is not due to their position as head of government but as a privilege of being current members of Her Majesty's Most Honourable Privy Council.

What are British prime ministers part of that grants them the title Right Honourable?
Her Majesty's Most Honourable Privy Council