Handle user login from an app to a website

I have an app and a web site that both can be logged into with the same login details. We have a web view in the app that needs to show a page from the web portal. Ideally management don't want to have to prompt the user to login in again when shown this web view.

I can easily do this by providing a link to the app that auto logs in for the user but I am concerned about these links being compromised.

What are people thoughts on how I can achieve this safely.

Note: I have full control of the web portal but little control over the app other than providing the initial link for the web view.