1. No one shall be held guilty of any criminal offence on account of any act or omission which did not constitute a criminal 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 the criminal offence was committed.
2. This Article shall not prejudice the trial and punishment of any person for any act or omission which, at the time when it was committed, was criminal according to the general principles of law recognised by civilised nations.
The natural injustice of this prohibition on retrospective punishment is embedded in English common law, and in this respect there is in close agreement with European Court rulings.
1.Rimmington’s 538 racist letters
In this case the defence argued that the legal meaning of “nuisance”. The great law lord,
Lord Bingham adopted a modern form of Article 7 saying:
There are two guiding principles (against retrospectivity): no one should be punished under a law unless it is sufficiently clear and certain to enable him to know what conduct is forbidden before he does it; and no one should be punished for any act which was not clearly and ascertainably punishable when the act was done. If the ambit of a common law offence is to be enlarged, it “must be done step by step on a case by case basis and not with one large leap”:
The House of Lords ruled that the legal meaning of “nuisance” encompasses activity such as Mr. Rimington’s.
R. v Rimmington, UKHL 2005 27, para. 33 http://www.bailii.org/uk/cases/UKHL/2005/63.html