Procurement: With regard to procurement legislation, is it mandatory to follow the existing regulations when a notice that has been issued is voluntary?

There are differing legal precedents with regard to this issue, with the European Court of Justice and British courts having taken a different tack on the issue. However, recent British rulings suggest that procedures that exist within the existing regulations always apply unless the authority makes it clear that it intends to follow different procedures. Thus, it would seem to be legally advisable to follow the legislation at all times.