Reformatio in peius (from Latin reformatio, 'change' - actually, 'improvement', and peius, 'worse') is a Latin phrase used in law meaning that a decision from a court of appeal is amended to a worse one.
The case law of the Boards of Appeal of the European Patent Office (EPO) does not allow a decision at appeal to put a sole appellant in a worse position. Hence in relation to appeals, the term "prohibition of reformatio in peius" basically means that a person should not be placed in a worse position as a result of filing an appeal. Thus, in general, EPO Boards of Appeal are prevented in opposition appeal proceedings from going beyond the request of a sole appellant to put it in a worse position that it was before it appealed.[1] The central case detailing this principle is G 4/93 consolidated with G 9/92.
Still under the case law of the boards of appeal of the EPO, the doctrine of reformatio in peius does not however apply separately to each point or issue decided, or to the reasoning leading to the impugned decision.[2]