An act or statute changing the law as to previous events or contracts. ‘Ex post facto’ laws are laws which voided and punished what had been lawful when done. [Phillips v Eyre (1870) 6 QB , 23,25]

The basic principle is that a person has a right to fair warning of that conduct which will give rise to criminal penalties and, therefore, the criminal quality attributable to an act should not be altered, after the fact, to the detriment of the accused. [Marks v. U.S., 430 US 188]

There is no funda­mental difference between retrospective and ex-post facto law. The former expression is used in respect of civil matters and the latter in respect of criminal matters which by their nature are more serious. [Nabi Ahmed vs. Home Secretary, Govt. of West Paki­stan, (1970) 22 DLR (SC) 20]