Service Oriented Computing (SOC) is a computing paradigm for the agile development of software based on the orchestration of loosely-coupled services. Services perform functions from the solution of simple atomic requests to execution of complex business processes. Nowadays, new services are created from the manual composition of basic Web Services. It is an arduous and susceptible to errors task. To avoid this problem, it is necessary the existence of discovery and automatic composition mechanisms. This paper presents the specification of a architecture and implementation of a prototype for the automatic composition of Web Services using Artificial Intelligence planning techniques and the WSMO ontological model. Moreover, the paper contains a discussion about design and implementation choices and further improvements.