S. 16 (3) (a) of the Copyright, Designs and Patents Act 1988 provides that references in Part I of the Act to the doing of an act restricted by the copyright in a work are to the doing of it in relation to the work as a whole or any substantial part of it. The question what is a "substantial part" for the purposes of this Act has been considered by the House of Lords on no less than three occasions.
In Ladbroke (Football) Ltd. v. William Hill (Football) Ltd.  1 W.L.R. 273 their lordships emphasized that the test of substantiality was qualitative rather than quantitative. A case that illustrates that point better than most is Warwick Film Productions Ltd. v Eisinger  1 Ch 508 where the volume of material that had been copied was considerable but such material did not constitute a substantial part of the claimant's work because it had itself been copied from an earlier work. The question then arises, as Lord Hoffmann observed at paragraph  of Newspaper Licensing Agency Limited v. Marks and Spencer Plc  UKHL 38 of what quality is one looking for? In his view it must be answered by referring to the reason why the work is given copyright protection. In literary copyright, where copyright is conferred irrespective of literary merit upon an original literary work, the quality relevant for the purposes of substantiality is the literary originality of that which has been copied. In an artistic work, it is the artistic originality of that which has been copied.
Different Types of Substantiality
In Designer Guild Limited v. Russell Williams (Textiles) Limited  UKHL 58 Lord Scott of Foscote said that s.16 (3) may come into play in two quite different types of case. One is where an identifiable part of the whole, but not the whole, has been copied such as a section of a picture, a sentence or two, or even a phrase, from a poem or book, or a bar or two of a piece of music. The other is where the copying has not been exact and where the copying has been with modifications such as the translation of a book or the adaptation of a novel. In the first type of case, the question whether the copying of the part constitutes an infringement depends on the qualitative importance of the part that has been copied, assessed in relation to the copyright work as a whole. His lordship endorsed the test proposed by Laddie, Prescott and Vitoria in the second edition of The Modern Law of Copyright and Designs at paragraph 2-108 of whether the infringer has incorporated a substantial part of the independent skill and labour of the original author.