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An important number of planning approaches have been proposed to tackle the problem of Web services composition which aims at selecting and inter-connecting services provided by different partners in response to client request. This process is beyond the human ability to analyse and generate the composition manually. Planning techniques are used widely in the literature to automatically compose web services but they don't scale up well. This weakness is due to the search space explosion caused by using the state-space or the plan-space planning techniques without take into account the distribution nature of web services domain. In a previous work, we proposed a decentralised multi-agent approach to solve the problem by associating a planning agent to each web services. Agent organises its services in a graph and respond to requests by its best partial plan extracted basing on a local heuristic. This approach is complete if the agents are independant. In this paper, we extend our previous approach by introducing a global heuristic to ensure the completeness and the optimality of the decentralised approach even if agents are dependent. The complexity of the new distributed algorithm is polynomial. This global heuristic of evaluating the best local plan avoids the wells by considering the intermediate local plans offered by other agents. To respond to a request, agents propose their best local plans basing on a distributed heuristic function. They then coordinate their partial plans by merging them to provide a global one for the submitted request.