Judges have a role in developing constitutional principles in applying the common law and interpreting legislation. The impact judges can have can be seen particularly in relation to decisions on the meaning of the European Communities Act 1972 and the Human Rights Act 1998.
The common law has helped to uphold the principle of the rule of law and importance of individual freedoms, especially from State interference.
Those acting as agents of the State must act lawfully, meaning they can only take actions which have a proper legal basis. This has been established to in common law and is related to the concept of the rule of law.
Entick v Carrington (1765)
Defendant had broken into the plaintiff's premises and seized some papers and the plaintiff brought an action for trespass. Defendant argued he had acted on a warrant authorised by a Government Minister.
Court decided the warrant was illegal as the Minister had exceeded his authority.
.. By the law of England, every invasion of private property, be it ever so minute, is a trespass. No man can set foot upon my property without my licence, but he is liable to an action though the damage be nothing....
The case illustrates how courts can provide a limitation to State power and is reflected in the current requirements for a warrant to be issued before the police may enter a house.
The fact disputes should be arbitrated by a court is another key element of the rule of law.
Case of Prohibitions (1607)
King James I placed himself in the position of judge for a dispute.
Court overturned the decision of the King and found he had no power to make such a decision. It was held that legal matters can only be properly heard by courts, subject to the rule of law.
Courts have developed the judicial review procedure in order to keep a check on executive and legislative power. In order to ensure the rule of law, judges can review whether government ministers, local authorities and public bodies have acted within their powers.
Judicial review can also be used to ensure the rules of procedural fairness are followed, which is linked to the concept of natural justice. This means decision makers must be free from bias and in a hearing there needs to be an opportunity for each party to be able to put forward their case and dispute their opponent's case.
Protection of Rights
The original premise of the common law is that a citizen is free to do as he wants unless the law clearly states that such conduct is prohibited. The Human Rights Act 1998 now provides additional positive protection of individual rights and freedoms.
Habeas corpus is a procedure which can be used to challenge the lawfulness of an individual's detention in a court. This right developed from a common law principle and is now enshrined in legislation. Article 5 (right to liberty and security of the person) of the European Convention on Human Rights is now incorporated into UK law by the Human Rights Act 1998.
A and Others v Secretary of State for the Home Department 
Defendant had a policy to detain several people, suspected of terrorist related activities, in Belmarsh Prison without charge.
Defendant's policy was found in breach of Article 5.
..It is not for the executive to decide who should be locked up for any length of time, let alone indefinitely... executive detention is the antithesis of the right to liberty and security of the person....
The supremacy of Parliament is a common law doctrine developed by the courts.
Jackson v Attorney General 
.. the supremacy of Parliament is still the general principle of our constitution. It is a construct of the common law. The judges created this principle....
Historically, the doctrine of Parliamentary Supremacy, has meant it is not usually possible to challenge the legality of an Act of Parliament. Although this situation has been modified slightly since the introduction of the Human Rights Act 1998 which allows courts to declare Acts incompatibile with certain rights in the European Convention on Human Rights.
Therefore, the courts main function is to interpret Acts of Parliament . It is their role to try to establish the intention of Parliament in passing an Act and rule on that basis. This is in accordance with the idea of the separation of powers, that defines the role of the judiciary as applying laws made by legislature.
However, despite courts being limited to the role of interpretation, decisions of the courts can still be constitutionally significant.
R v. Secretary of State for Transport ex parte Factortame Limited (No 2) 
House of Lords effectively suspended the operation of provisions in the Merchant Shipping Act 1988 which conflicted with EU law, on the basis that the European Communities Act 1972 gave EU law primacy.
.. whatever limitation of its sovereignty Parliament accepted when it enacted the European Communities Act 1972 was entirely voluntary....
There is no formal distinction between constitutionally significant legislation and other statutes. More recently, it has been suggested some should be considered constitutional statutes which the courts should presume Parliament does not intend to repeal, unless it does so expressly.
Thoburn v Sunderland City Council 
The case classified the European Communities Act 1972 as a
Lord Justice Laws:
.. [constitutional Acts are those which] ( a) conditions the relationship between citizen and State in some general, overarching manner, or (b) enlarges or diminishes the scope of what we would now regard as fundamental rights....
Presumptions of Statutory Interpretation
In determining the meaning of constitutional statutes the courts will make some general presumptions.
The separation of powers leads to the taxation presumption, that taxes must be authorised by Parliament and the access to court presumption, that it is role of courts to decide legal disputes. The rule of law leads to the presumption against uncertain laws and retrospective penalties, that there is no punishment except for a distinct breach of the law.