Assume a criminal offence occurred in the past (for example 2015), before a new law was passed at a future date (for example 2016).

What law would be referred to by the judge when it comes to sentencing the offender? I think there is a legal term for this in Latin but I cannot find it.

My question is focusing on the United States and the United Kingdom.

I have done some research, and it appears that ex de facto law is not allowed in the United Kingdom to follow directions given by the European Union. However, I am not sure if this covers my question, as it is for actions that were previously legal, whereas my question is about actions that were illegal and then had changes made to them.

  • 2
    It's ex post facto.
    – phoog
    Feb 4, 2023 at 0:17
  • @phoog So both the UK and USA would have to punish an offender based on the law at the time the offence was committed? Feb 4, 2023 at 0:19
  • @user5623335 there's nothing expressly preventing the UK parliament from criminalizing past actions, or punishing them more severely following amended law, however the only recent times it's come up is in tax law, and the ECHR might currently limit it for criminal law.
    – origimbo
    Feb 8, 2023 at 17:48
  • AFAIK the European Convention on Human Rights does prevent new statutes from applying to past actions that were legal at the time, but I am unsure what happens when something that would have been for example 6 months in prison raises to 10 years. Sorry to overexaggerate it is just for clarity. Feb 9, 2023 at 23:48

1 Answer 1


An ex post facto law criminalizes conduct after the fact that was legal at the time, something that is prohibited under U.S. constitutional law. But, as the question states:

my question is about actions that were illegal and then had changes made to them.

If something is a crime in 2015 and this criminal law is violated, and the conduct is then legalized in 2016, the conduct committed while it was a crime does not cease to be punishable as a crime and may be punished criminally.

Often a judge would consider the fact that the conduct was later legalized when evaluating the seriousness of the crime at a post-legalization sentencing, but a judge is not required to do so.

A significant number of pardons and commutations of criminal sentences by Presidents and Governors in the U.S. involve people convicted of crimes for conduct that is now legal or is now punished less severely. But pardons and commutations are purely discretionary.

As an aside, France has constitutional protections that give newly lenient treatment of crimes retroactive effect, but the United States does not.

  • That was very helpful and interesting. However, what if in 2015 the worst punishment was a fine (like $3,000) but in 2016 it became a felony with 5 years prison. Are you saying if the criminal broke the law in 2015 they would get the fine? Feb 4, 2023 at 0:27
  • I'll think about that fact pattern before responding. It hadn't occurred to me that this was what you were asking.
    – ohwilleke
    Feb 4, 2023 at 0:54
  • 2
    @user5623335 Correct. The rule against ex post facto laws includes laws that increase the severity of the punishment for a crime.
    – cpast
    Feb 4, 2023 at 2:46

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