The Cabinet Office is a ministerial department of the Government of the United Kingdom. It is responsible for supporting the prime minister and Cabinet. It is composed of various units that support Cabinet committees and coordinate the delivery of government objectives via other departments. As of December 2021,^[update] it had more than 10,200 staff, mostly civil servants, some of whom work in Whitehall. Staff working in the Prime Minister's Office are part of the Cabinet Office.

The Cabinet Office's core functions are:

The Cabinet Office has responsibility for the following at the UK national level:

The department was formed in December 1916 from the secretariat of the Committee of Imperial Defence under Sir Maurice Hankey, the first Cabinet Secretary.

Traditionally the most important part of the Cabinet Office's role was facilitating collective decision-making by the Cabinet, through running and supporting Cabinet-level committees. This is still its principal role, but since the absorption of some of the functions of the Civil Service Department in 1981 the Cabinet Office has also helped to ensure that a wide range of Ministerial priorities are taken forward across Whitehall.

It also contains miscellaneous units that do not sit well in other departments. For example:

In modern times the Cabinet Office often takes on responsibility for areas of policy which are the priority of the Government of the time. The units that administer these areas migrate in and out of the Cabinet Office as government priorities (and governments) change.

Leaders of the Houses of Commons and Lords, supported by the Cabinet Office, are as follows: