Automated composition of Web services has received much interest in the last decade, as it supports B2B applications. It aims at selecting and inter-connecting services provided by different partners in response to client requests.Planning techniques are used widely in the literature to describe Web services composition problem. However, since Web services proliferate day after day, classical planners are no longer well suited to compose Web services in a reasonable time. This weakness is due to the explosion of the search space caused by the large number of services and the broad range of data exchanged among services. In this paper, we propose a decentralised multi-agent approach to solve the Web services composition problem at runtime. Our model consists of a set of Web service agents where each agent has a set of services organised in a graph. Responding to a request, agents propose partial plans which are partial paths in the graph, then they coordinate their partial plans to provide the best global plan for the submitted request. The analysis of the complexity and results of the implementation show the ability of our approach to scale up when compared to the of state-of-the-art techniques for automated web service composition.