Properly titled ‘Her Majesty's Principal Secretary of State for...’, Secretaries of State are the senior Ministers of Government, and take an oath of office on appointment.
A Secretary of State is typically the sole head of a Department or Ministry, chairs the Departmental Board, and bears ultimate responsibility for the actions of their Department. A Secretary of State is a voting member of Cabinet, and will be appointed a Privy Councillor for this purpose.
There is only one Secretary of State, the position originating in medieval times as the principal minister of the Sovereign. Today the position is held 'in commission' (i.e. the duties performed by a group of people, rather than an individual). The title ‘Her Majesty's Principal Secretary of State for …’, reflects the responsibilities of the single office being divided between those ‘Principal Secretaries.’
The singularity of office means any Secretary could therefore act on another’s behalf, as Home Secretary Sir John Simon described in 1936:
“I happened to be Secretary of State for Home Affairs at the time when Lord Kitchener made his last fatal journey from this country, and just before he sailed from these shores I had a message from the War Office asking me whether I would sign his papers until he came back. I continued to give the formal signature which it is legitimate for any Secretary of State to put upon the papers of any other Secretary of State, and I did so until the news came of his death.”
In legislation, references to the powers of a Secretary of State will be to the singular office ‘the Secretary of State’, and not refer to a more specific Principal Secretary.
For example, the power for UK intelligence services to perform otherwise illegal activities is governed by Section 5 of the Intelligence Services Act 1994, which states:
The administration of which Secretary of State is responsible for powers provided in legislation is an administrative matter for Government. In the case of the Security and Intelligence Agencies:
Though Ministers will always be held ultimately responsible for actions taken under their authority, the 'Caltona doctrine' allows civil servants and junior Ministers to act on behalf of the Secretary of State. However, in some cases this may be explicitly prohibited by legislation. Reading on to Section 6 of the Intelligence Services Act 1994 the procedure for issuing warrants under Section 5 states:
Therefore, warrants for MI5, MI6 and GCHQ must be issued personally by the Secretary of State, and only by officials acting on their behalf in exceptional circumstances.