Provision of adaptive access control is key to allowing users harness the full potential of ubiquitous computing environments. In this paper, we introduce the M-Zones Access Control (MAC) process, which provides user-centric attribute-based access control, together with automatic reconfiguration of resources in response to the changes in the set of users physically present in the environment. User control is realised via user-specified policies, which are analysed in tandem with system policies and policies of other users, whenever events occur that require policy decisions and associated configuration operations. In such a system users' policies may habitually conflict with system policies, or indeed other users' policies; thus, policy conflict detection and resolution is a critical issue. To address this we describe a conflict detection/resolution method based on a policy precedence scheme. To illustrate the operation of the MAC process and its conflict detection/resolution method, we discuss its realisation in a test bed emulating an office-based ubiquitous computing environment.