Ex post facto law:
An ex post facto law is a law that retroactively changes the legal consequences (or status) of actions that were committed before the enactment of the law. In criminal law, it may criminalize actions that were legal when committed; it may aggravate a crime by bringing it into a more severe category than it was in when it was committed; it may change the punishment prescribed for a crime, as by adding new penalties or extending sentences; or it may alter the rules of evidence in order to make conviction for a crime likelier than it would have been when the deed was committed. Retroactive laws are not preferred because it infringes upon individuals autonomy, and leads to legal uncertainty. For example: The legislative has made abortion illegal but they cannot take action again abortion clinics that have performed abortions before the law was made. They can say "from this day forward" but they cannot go back in time and persecute people for it.So legislative can’t enact ex post facto law.
Position in Bangladesh Constitution:
Article 35(1) of the Bangladeshi Constitution that prohibits conviction or sentence under ex post facto law, “No person shall be convicted of any offence except for violation of a law in force at the time of the commission of the act charged as an offence, nor be subjected to a penalty greater than, or different from, that which might have been inflicted under the law in force at the time of the commission of the offence.”
Position in Indian Constitution:
Article 20(1) of the Indian Constitution in the following words: “No person shall be convicted of any offence except for violation of a law in force at the time of the commission of the act charged as an offence, nor be subjected to a penalty greater than that which have been inflicted under the law in force at the time of commission of the offence.”
Position in US Constitution:
In the United States of America, the federal government is barred from passing Ex Facto Laws; Clause 3 of Article 1, section 9 of the United States Constitution prohibits the federal government from passing such laws.
Position in UK Constitution:
Retrospective criminal laws are prohibited by Article 7 of the European Convention on Human Rights, to which the United Kingdom is a signatory, but several noted legal authorities have stated their opinion that parliamentary sovereignty takes priority even over this.[22][23] For example, the War Crimes Act 1991 created an ex post facto jurisdiction of British courts over war crimes committed during the Second World War.
Universal Declaration of Human Rights and related treaties:
Article 11, paragraph 2 of the Universal Declaration of Human Rights provides that no person be held guilty of any criminal law that did not exist at the time of offence nor suffer any penalty heavier than what existed at the time of offence. It does however permit application of either domestic or international law. Very similar provisions are found in Article 15, paragraph 1 of the International Covenant on Civil and Political Rights, replacing the term "penal offence" with "criminal offence".
