The doctrine of implied repeal is a concept in constitutional theory which states that where an Act of Parliament or an Act of Congress (or of some other legislature in a common law system) conflicts with an earlier one, the later Act takes precedence and the conflicting parts of the earlier Act are repealed (i.e., no longer law). This doctrine is expressed in the Latin phrase "leges posteriores priores contrarias abrogant".
Implied repeal is to be contrasted with the express repeal of legislation by the legislative body.
Under United States law, "implied repeal" is a disfavored doctrine. That is, if a court can reconcile the two statutes with any reasonable interpretation, that interpretation is preferred to one that treats the earlier statute as invalidated by the later one.
Constitutional statutes in the United Kingdom: not subject to implied repeal
In the 2002 English case Thoburn v Sunderland City Council (the so-called 'Metric Martyrs' case), Lord Justice Laws expressed the view that some constitutionally significant statutes held a higher status in UK law and were not subject to the doctrine of implied repeal. The case specifically dealt with s.2(2) of the European Communities Act, but in his judgment Lord Justice Laws also named the Parliament Act and the Human Rights Act as 'constitutional statutes' and therefore not subject to the doctrine of implied repeal. Constitutional statutes can still be expressly repealed if Parliament so wishes.
A decade later in 2012, in a case before the Supreme Court of the United Kingdom, BH v The Lord Advocate (Scotland), Lord Hope said in paragraph  of the judgment that “the Scotland Act can only be expressly repealed; it cannot be impliedly repealed; that is because of its ‘fundamental constitutional nature’.”
- Free Life Commentary No 63
- See Stop Youth Addiction, Inc. v. Lucky Stores, Inc., 950 P.2d 1086, 1096 (Cal. 1998) (recalling that to overcome the strong presumption against implicit repeal the two provisions must be so inconsistent that they cannot have concurrent operation).
-  UKSC 24
- Adam Perry and Farrah Ahmed: Are Constitutional Statutes ‘Quasi-Entrenched’? Blog of the UK Constitutional Law Association