In ubiquitous computing, user needs may opportunistically emerge along the variation of the context of use. Thus, there is a need for dynamically composing interactive systems. An interactive system is made of a functional core and a User Interface (UI). This paper deals with the composition of UIs to support opportunistic user needs in a given context of use (user, platform, environment). A state of the art about UI composition shows the originality of our work: the composition of a task model. The composition of the concrete UI is then delegated to a toolkit of interactors defined at the task level. The composition of the task model is done by automated planning. The work shows that current planners do not fulfill Human Computer Interaction (HCI) requirements. Therefore, a specific planner has been developed to compose UIs. This planner is used in Compose, our proof of concept. The work is original in two points: first, by the high level of abstraction the composition is performed; secondly, by the use of automated planning in HCI.