This paper presents a distributed authorisation model suitable for use in a web service framework where multiple parties are involved in performing a particular transaction. The authorisation model uses a third party authorisation service that checks users or services' credentials against a set of authorisation policies. A traditional service provision model does not scale well for such transactions. The proposed model uses a hardware security appliance to deliver the service to the most appropriate site involved in the transaction. The authorisation model supports a multi-party session so that authorisation policies can be checked and built as part of the web service composition process. 14 Pages