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There's no setting in the phone that allows you to force portrait mode. Even if you turn off auto rotation under Settings -> Display an app may switch to landscape. many games do this for example but if you place the phone in the docking station it will also rotate.
There are some 3:rd party apps that can force portrait like https://play.google.com/store/apps/details?id=com.googlecode.eyesfree.setorientation but if you try to run an app that's designed for landscape it might not work properly.
I second this. It's a shame I can't lock portrait mode. I can do it on all other Android devices, but not Sony. Please include this into the new update.
Try this: Long press on a blank spot on the home screen > Home Settings > Toggle Auto rotation off