An ex post facto law is a law which imposes penalty retroactively, that is, upon acts already done or which increases the penalty for such acts. As for example, suppose in 1955 a person does an act which is not, then lawful, In 1956, a law is passed making that act a criminal offence and seeking to punish that person for what he did in 1955. Or suppose, punishment prescribed for an act in 1955 Is six months, but in 1956, the punishment is increased for a year and is made applicable to the acts committed before 1956. These are examples of ex post facto laws.
In India, Article 20 (1) provides the necessary protection against an ex-post facto law. Article 20 (1) has two parts. Under the first part, no person is to be committed of an’ offence except for violating a “law in force” at the time of commission of the act charged as an offence.
A person is to be convicted for violating a law in force when the act charged is committed. A law enacted later, making an act done earlier (not an offence when done) as an offence, will not make the person liable for being convicted under it.
In Prahlad Krishna v. State of Bombay, AIR 1952 Bom. 1, it has been held that immunity is thus provided to a person from being tried for an act under a law enacted subsequently, which makes the act unlawful.
The immunity extends only against punishment by courts for a criminal offence under an ex post facto law and cannot be claimed against preventive detention or demanding a security from a press under the press law, for acts done before the relevant law has been passed.
The second part of Article 20 (1) immunizes a person from a penalty greater than that which he might have been subjected to at the time of his committing the offence. Thus, a person cannot be made to suffer more by an ex post facto law than he would be subjected to at the time he committed the offence.
This clause applies to punishment for criminal offences only. In Kedar Nath v. State of West Bengal, AIR 1953 S.C. 404, the accused committed an offence in 1947 under Prevention of Corruption Act which then prescribed a punishment of imprisonment or fine or both. In 1949, by an amendment of the law, the punishment prescribed was enhanced by a further fine to the equivalent to the amount the money found to have been procured by the offender through his offence. The Supreme Court held that the enhanced punishment prescribed in 1949 could not be applicable to the offence committed in 1947 because of Article 20 (1).
In this way we find that under Article 20, all that has to be considered is whether the ex post facto law has imposed a penalty greater than that which might be inflicted under the law in force at the time of commission of the crime.