Authentication MVC 3 - WCF Question

The solution I'm building consists of:
- Presentation layer: MVC 3; Controller calls WCF REST Services, Model  = presentation model (domain model, repository, ... sits in WCF part)
- WCF REST Services

So far so good this architecture ... but I'm completely stuck regarding Authentication/Authorization (I would like using Forms Authentication). Users can authenticate in the MVC app. But how should I solve the additonal problem about authenticating to the WCF REST services? If on the WCF side there's no authetication someone can directly access business logic, data, ...?

Thanks.
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