His Majesty's Most Loyal Opposition

His Majesty's Most Loyal Opposition (called Her Majesty's Most Loyal Opposition when a queen reigns), often abbreviated to HM Most Loyal Opposition, commonly known as the Official Opposition, and colloquially The Opposition, is the main official political opposition in the United Kingdom to His Majesty's Government. This is usually the largest political party in the House of Commons that does not form part of the government or a governing coalition.

Source: Wikipedia — His Majesty's Most Loyal Opposition (CC BY-SA 4.0)