Virtual organisations are non-conventional structures aiming at sharing the same goals, developing common practices or creating wealth in a collaborative manner. Accordingly, competence issues are important topics to be addressed in order to ensure coherence between the different objectives of entities taking part in a virtual organisation to guarantee an efficient use of available resources. This paper introduces the principles for modelling competence-based virtual organisations by using a unified enterprise competence modelling language (UECML). Our proposed modelling approach is inspired from previous related works mainly in competence-based enterprise modelling. The language provides a neutral interface to virtual organisation modelling based on competences, taking into account the several roles and entities participating in a virtual organisation. Therefore, providing constructs covering processes, resources, competences, and virtual organisation (VO) entities. A case study of the Swiss virtual network Virtuelle Fabrik with regards to the project Abfallhai is presented and discussed, showing how the developed modelling language is adapted to model virtual organisations.