Almost certainly structural, quite likely the original timber windows included timber panels that were supporting the brickwork.
Is that window downstairs- if yes, get a spirit level on the floor above it. If the floor is sinking towards the window you need to chop the plasterwork out and see what's underneath. If the floor isn't sinking then you'd be wise to have an investigate but with less urgency (common Edwardian technique i've found is a biggish timber as a lintel, then a couple of courses of brick and below that a smaller timber, originally stabilised with the timber door or window frame. The main timber may be ok, the smaller timber may be sagging due to uPVC replacement)
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