Page "Head of state" Paragraph 68

The head of state accredits ( i. e. formally validates ) his or her country's ambassadors, High Commissioners or rarer equivalent diplomatic mission chiefs ( such as papal nuncio ), through sending formal Letter of Credence to other heads of state and, conversely, receives the letters of their foreign counterparts.

Without that accreditation, they cannot take up a role and receive the highest diplomatic status.

However, there are provisions in international law to perform the same diplomatic functions, or at least part of them, such as accrediting, with a lower title with the head of government, or functioning within another mission.