Deferring a PhD to pursue a masters after accepting it. Advice? I have accepted a fully funded PhD offer from a very good university. In the meanwhile I have managed to secure a masters offer from another very good university. (I applied to both at the same time, but was given a masters offer a week after the funding acceptance deadline for my PhD)

The opportunity to attend two of the worlds best universities is a huge decision that will affect my future in a very significant level. 

Especially given that if something where to happen in the 4 years of my PhD (change in supervisor's or personal circumstances) which would prevent me from completing the PhD, I would still have a Masters under my belt if I pursue both opportunities. 

I would therefore like to defer my funded PhD start date to next year, in order to do the masters. Rather than do one or the other.

I would like your opinion and I would also like to ask for possible consequences of such an action, and how best to approach it.

(I note that suspensions of studies can be available for exceptional circumstances(including work/career opportunities). I would class the masters as a career opportunity, given that is at one of the world's best university and career focused with many industry links.) 

How do I very nicely request to defer my studies by one year to pursue a masters opportunity - which will enhance my career prospects greatly?
Essentially, you have to choose between the two; you won't be able to do this the way you propose in your question.

There are usually only a few allowed reasons why a school will let you defer your enrollment: to fulfill a service obligation for a scholarship; to participate in a special program, such as a Rhodes, Marshall, or Fulbright scholarship; or if you have exigent medical circumstances that require you to postpone the start of your studies.

One reason that is generally not allowed is for you to study somewhere else. Just because you'd classify it as a "career opportunity," the PhD program to which you have been admitted will see it differently. They will make you decide between the two programs: if you want to enroll in the master's program, you'd have to give up your PhD admission spot, and reapply for the appropriate term in which you'd be able to start following your master's program.