Being E-commerce one of the most critical Internet application, it is fundamental to employ technologies which guarantee not only secure transactions but also an adequate quality of service. In this paper we present a solution to this problem based on an extension of the emerging Web Service technology. In particular we introduce a new Web Service discovery protocol that extends standard UDDI capability by adding: (i) the discovery of Web Services at run-time supporting environment re-configurations, (ii) security access control to Web Services and (iii) a mechanism for distributing service invocations among several Web Services implementing (at different efficiency levels) the same task. Discovery at run-time is realized by dynamically resorting to the discovery protocol (a Web Service itself) every time a service is invoked, access control is obtained by employing symmetric and asymmetric keys, while the different efficiency levels of service implementations are represented via w...