I'd prefer not to unless it was an absoloute last resort.
It makes it a complete b'stard to replace the back box when it gets damaged, and it is IMO even less acceptable if the cables are fed from a different circuit as there is no real way to know for certain that those cables are not live before you snip them.
(It is even worse if they are on different phases)
Can you not use a T box and mount the socket upto this?
Also if you have a few circuits running through the back box then it may end up being too rammed in there to screw the socket front on without crushing / nicking one of the cables.
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