§ Mr. Mitchell
To ask the Secretary of State for Foreign and Commonwealth Affairs in what circumstances persons who are not members of Her Majesty's Privy Council are authorised to sign treaties on her behalf. 
§ Mr. Doug Henderson
Most treaties are signed with Governmental Full Powers either by Ministers or Ambassadors. Membership of the Privy Council is not a factor in the issuing of Full Powers.
The Vienna Convention on the Law of Treaties provides that Heads of State, Heads of Government and Ministers for Foreign Affairs are eligible to sign treaties by virtue of their office. Anyone else signing a treaty in articled form requires a formal document known as a Full Power.
In the United Kingdom, the Treaty-making power is vested in the Crown as part of the Royal perogative. The exercise of that power is the concern of my right hon. Friend the Secretary of State for Foreign and Commonwealth Affairs.
The United Kingdom has three types of Full Power: Queen's General Full Powers, which are signed by Her Majesty The Queen and empower the Secretary of State for Foreign and Commonwealth Affairs, FCO Ministers of State, FCO Under-Secretaries of State to sign any treaty; and the UK Permanent Representatives to the UN and the EC to sign treaties in their respective fields. Queen's Special Full Powers, which are issued for the signing of a specific treaty drawn up between Heads of State. (iii) Governmental Full Powers, which are signed by the Secretary of State for Foreign and Commonwealth Affairs and are issued for the signing of specific inter-governmental and inter-state treaties.