Yep - under RIPA Part III
A couple of examples from a quick Google:
- http://www.theregister.co.uk/2009/11/24/ripa_jfl/
- http://www.bbc.co.uk/news/uk-england-11479831
In both cases I think there wasn't any doubt they had the passwords. I'm not aware of any cases where there was doubt whether the person _could_ unlock the files, or where that was a plausible defence.