Unit Testing Web Services - HttpContext

I want to write unit tests for a web service. I create my test project, reference my web project (not service reference, assembly reference), then write some code to test the web services - they work fine. However, there are some services which make sure the user is logged in to the web application by using HttpContext.Current.User.Identity.IsAuthenticated.

In the context of the tests, there is no such thing as HttpContext, so the tests always fail. How should these kinds of web services be unit tested?