The MHRA Style Guide Online
A Handbook for Authors and Editors • Third Edition
From Chapter 9, 'Quotations and Quotation Marks'
9.7 Copyright and Permissions
It is the responsibility of an author to obtain any necessary permission for quotation of copyright material. The author should ensure that permission to reproduce material in all territories and all media (e.g. print and electronic) is granted.
Normally it is unnecessary to seek permission for the quotation of brief passages in a scholarly work, but it is not possible to give a definitive ruling to indicate when it is necessary to seek permission: copyright laws are not the same in all countries, and publishers hold differing views on the subject. If in doubt, authors should consult their editors.
In general it may be said that the length of the quoted passage and the use to which it is put should be fair to the author and publisher of the work quoted in that nothing is done to diminish the value of their publication.
Complete items such as tables, illustrations, and poems must not be reproduced without permission. (See also 1.3.11.)
On private correspondence, see also 11.2.10 below.
The British Academy and the Publishers Association have published Joint Guidelines on Copyright and Academic Research at <http://www.britac.ac.uk/policy/joint-copyright-guide.cfm>.