Separation of powers is one of the core principles of the UK constitution. The doctrine is based on the idea that the legislative, executive and judicial powers of government should be exercised by separate institutions in order to prevent absolute power being held by any individual or group.
Montesquieu defined government as having three distinct functions, the legislative function (making of the rules), executive function (governing according to the laws) and judicial function (resolving disputes according to law). He argued each function should be exercised by independent institutions.
Montesquieu suggested the separation of powers enabled liberty,
.. when the legislative and executive powers are united in the same person... there can be no liberty... again, there is no liberty, if the judiciary power be not separated from the legislative and executive.... . He argued
.. liberty is a right of doing whatever the laws permit... (Spirit of the Laws (1748)).
In the UK context, Parliament exercises the legislative powers, Government the executive function and there is an independent judiciary.
Locke linked the concept of separation of powers to the idea of liberty. He argued that in order for the effective protection of individual liberties, the power of government
.. ought to be exercised by established and promulgated laws, that both the people may know their duty, and be safe and secure within the limits of the law, and the rulers, too, kept within their due bounds, and not be tempted by the power they have in their hands to employ it to purposes, and by such measures as they would not have known, and own not willingly... (Two Treatises of Government (1689)).
Locke is particularly concerned that there are appropriate checks and balances on the power of the executive. He puts forward
.. it may be too great temptation to human frailty, apt to grasp at power, for the same persons who have the power of making laws to have also in their hands the power to execute them, whereby they may exempt themselves and execution, to their own private advantage....
There are tensions within the doctrine and its relationship with core constitutional values. The doctrine highlights the distinction between separation and independence. The functions of a State need to be carried out by separate institutions which can make decisions without undue influence from another. However, as all perform a role in creating and upholding law then absolute independence is not desirable.
For example, if the laws made by the legislature are not supported by the executive then individuals are unable to rely on the laws and the purpose of the separation is defeated. Therefore, there is an implicit need for coordination between legislative and executive functions. Equally, if a court was independent to the extent that it did not feel constrained to implement the laws made by the legislature then there would be no legal certainty and the rule of law would be undermined.
Therefore, it should be recognised that there is an interrelation between the functions of executive, legislature and judiciary. Parliamentary sovereignty allows Parliament to set a framework of law for Government. This can be consistent with the principle of separation of powers and Locke noted that
.. there can be but one supreme power, which is the legislative, to which all the rest are and must be subordinate....