Règle correspondante
Practice Relating to Rule 101. The Principle of Legality
Pakistan’s Anti-Terrorism Act (1997) states:
Punishment for terrorist act committed before this Act
Where a person has committed an offence before the commencement of this Act which if committed after the date on which this Act comes into force would constitute a terrorist act there under he shall be tried under this Act but shall be liable to punishment as authorized by law at the time the offence was committed. 
Pakistan, Anti-Terrorism Act, 1997, Section 38.
In its judgment in the Baluch case in 1968, the Supreme Court of Pakistan stated:
It is well-settled that a law is not to be given retrospective effect unless it is expressly or by necessary intendment made retrospective … In the present case, what was under challenge in the High Court, was the validity of the detention order passed on the 11th August 1966, which was made before the present amendments came into force. The validity of that order has, of necessity, therefore, to be judged on the basis of the law prevailing on that day. 
Pakistan, Supreme Court, Baluch case, Judgment, 24 May 1968.
In his Separate Opinion in the Shahida Faisal case before Pakistan’s Lahore High Court in 2000, Judge Mian Nazir Akhtar stated:
12. The Universal Declaration of Human Rights incorporates a provision against retrospectivity of a penal offence. Its Article 11 reads as under:–
“No one shall be held guilty of any penal offence on account of any act or omission which did not constitute a penal offence under national or international law, at the time when it was committed. Nor shall a heavier penalty be imposed than the one that was applicable at the time when the criminal offence was committed.”
13. The question of retrospectivity of a penal offence came up for consideration before the Supreme Court of Pakistan in the case of Nabi Ahmad and another v. The Home Secretary, Government of West Pakistan, Lahore and 4 others (PLD 1969 SC 599). It was held that statutes are presumed to be applicable to cases and facts coming into existence after their enactment unless there be clear intention to give them retrospective effect. In this judgment it was clarified that the expression retrospective statute is relatable to a civil matter whereas the expression ex post facto legislation pertains to criminal matters. The Court referred to the nature and meaning of ex post facto law as follows: –
“An ex post facto law is one which makes criminal and punishes an act which was done before the passage of the law and which was innocent when done, aggravates a crime or makes it greater than it was when committed, changes the punishment and inflicts a greater punishment than was prescribed when the crime was committed, or, alters the legal rules of evidence and receives less or different testimony than was required to convict at the time the offence was committed. Further, an ex post facto law may be one which, assuming to regulate civil rights and remedies only, in effect imposes a penalty or the deprivation of a right for something which when done, was lawful, deprives persons accused of crime of some lawful protection or defence previously available to them, such as the protection of a former conviction or acquittal, or of a proclamation of amnesty, or generally, in relation to the offence or its consequences, alters the situation of an accused to his material disadvantage.” (Taken from Corpus Juris Secundum, Vol. 16-A, Article 435).
The Court was pleased to hold that: –
“Rights of the parties arising from facts which come into existence before the passing of a statute should be presumed to be unaffected by it unless it is expressly or by necessary implication made retrospective. The full significance, and implications, of the protection cannot be fully appreciated, unless we discover its reason. This is not a statutory protection, yet the principle has by virtue of a presumption of fair play effectively checked encroachments on existing rights by the all powerful British Parliament, unless they were found to have been clearly and unambiguously so intended. The origin of this presumption is to be found in the conscientious abhorrence that all just men have for the injustice that is inherent in changing the legal implications of a situation to the disadvantage of those who would otherwise benefit by a right which existed at the time of the change. As a manifestation of more or less, a natural or instinctive sense of justice, or perhaps gut instinctive repugnance to what one feels to be injustice, the Courts have held that laws do not ‘impose new liabilities in respect of events taking place before their commencement’. Since the above way of thinking is the consequence of a sense of aversion for injustice, it is immaterial ‘whether the law is changed before the hearing of the case at first instance or whilst an appeal is pending’. One more consideration which appeals is that law-abiding members of society regulate their lives according to the law as it exists at the time of their actions, and they expect the law to be steadfast and reliable. They assess and weigh the consequences according to the demands of existing system of law, including the requirements implicit in the existing system of law, and are entitled to feel cheated if the law later lets them down by taking away or reducing their rights, or increasing their burdens. The time at which a presumption arises against retrospectivity is to be determined by the circumstances which call for protection against injustice. The differences in the manifestations of this deed are mere matters of detail. The need may arise before the commencement of proceedings with reference to the time at which a cause of action arose, or an innocent deed was done. This happens if, for instances, a law is made to eliminate that cause of action or to make that innocent act punishable. It may also arise with reference to the time at which a new law was enforced during the pendency of a proceeding. Such an occasion can arise if, for instance, the right of appeal is abolished after the institution of a proceeding. So also it may arise with reference to the time at which a proceeding, whether civil or criminal was concluded by a decree, conviction or acquittal, and then a law was passed taking away the right of appeal against the decree, conviction or acquittal, when no proceeding was pending. When we think of such an injustice, we are really thinking of the adverse effect of the new law on vested rights.”
14. Justice Blackstone explained ex post facto laws in the case of Philips v. Eyre (1870) 6 QB 1 in the following words:–
“After an action, indifferent in itself, is committed, the Legislature then for the first time, declares it to have been a crime, and inflicts a punishment upon the person who has committed it. Here it is impossible, that the party could foresee that an action, innocent when it was done, should be afterwards converted to guilt by a subsequent law; he had therefore, no cause to abstain from it; and all punishment for not abstaining must of consequence be cruel and unjust”.
15. The prohibition against ex post facto legislation was contained in the 1956 and 1962 Constitutions of Pakistan. Article 6 of the 1956 Constitution of Pakistan containing prohibition against conviction for an offence which was not an offence when committed is materially the same as Article 20 of the Indian Constitution. However, Article 9 of 1962 Constitution is substantially the same as Article 12 of the Constitution of Pakistan, 1973 which reads as under:–
“Protection against retrospective punishment.–
(I) No law shall authorise the punishment of a person–
(a) for an act or omission that was not punishable by law at the time of the act or omission; or
(b) for an offence by a penalty greater than; or of a kind different from; the penalty prescribed by law for that offence at the time the offence was committed.”
The word “law” used in sub-clauses (a) and (b) of clause (1) of the above-quoted Article means law in operation at the time of the commission or omission charged as an offence. A law in force cannot be equated with a law “deemed to be in force” through ex post facto legislation. The term “law” used in the Article postulates actual and not notional existence of the law at the relevant time. 
Pakistan, Lahore High Court, Shahida Faisal case, Separate Opinion of Judge Mian Nazir Akhtar, 14 June 2000, §§ 12–15.
In 2001, in its second periodic report to the Committee on the Rights of the Child, Pakistan stated the following principle from its Constitution: “Penal sanctions shall not be applied without prior legislation, and penal laws shall not be applied retrospectively (art. 12).” 
Pakistan, Second periodic report to the Committee on the Rights of the Child, 11 April 2003, UN Doc. CRC/C/65/Add.21, submitted 19 January 2001, § 366.