A method and apparatus for enabling a user to access a service provider is described. The user sends a request from a browser to a proxy server. The proxy server modifies the request by adding data (such as a URL) relating to a location of an identity provider able to provide user credentials for the user and forwards the modified request to the service provider. The modification of the access request may occur before the request is sent to the service provider or in response to an authentication request from the service provider. The data relating to the location of the identity provider may be provided as a header (e.g. an http header).