Can one copy passages from a Master's thesis into a PhD thesis?

I wrote a Master's thesis a couple of years ago (which is available to view from my University's online service so it's out there...) and now doing a PhD. Some passages from the Master's thesis is completely relevant to what I am researching and can include it as part of the introduction/literature review.

Is it frowned upon if I copy chunks from my masters into my PhD thesis?

My work is not published and I am studying in a UK university (if it makes a difference).