The MHRA Style Guide Online
A Handbook for Authors and Editors • Third Edition
From Chapter 5, 'Punctuation'
5.7 Ellipses and Square Brackets
In quotations, points indicating an ellipsis (i.e. the omission of a portion of the text) should be enclosed within square brackets:
Her enquiries […] were not very favourably answered.
This practice makes it possible to distinguish between points indicating an ellipsis and points that occur in the original, as in the following quotation from Samuel Beckett:
Will you never have done … revolving it all?
and in this one from Tom Stoppard:
Well, he’s completely mad, of course. They all are….
The original punctuation is retained when it is possible to do so:
When, in the course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another […], a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.
Outside the hut I stood bemused. […] It was still morning and the smoke from the cookhouse rose straight to the leaden sky.
When the beginning of a sentence is omitted, the first word following the ellipsis can be capitalized even if it does not have a capital in the original:
For the rest of the evening, von Igelfeld considered his response. […] He could just ignore the article altogether.
(In the original, the passage abbreviated ends ‘And finally, he could just ignore the article altogether’.)
One may also indicate a change of case in square brackets:
Mrs Bennet felt that ‘[t]his was invitation enough’.
‘[A] young man of large fortune’ had taken Netherfield.
See too 9.6 on omissions within quotations.