Reading novels and stories written by English and American writers one can find many idioms. But the most authors often change the structure and the contextual meaning of the national idioms transforming them according to the stated objectives. Such set-expressions are called modified, or transformed, idioms. They are deliberate, creative and innovative forms of the traditional idioms used in folk speech. The object of the article is the English national idioms which were modified by the British writers. The subject of the article is the stylistic and semantic peculiarities of the modified idioms in the British literary discourse. The objective of the paper is the semantic description of the modified idioms used by the British authors in the prose. In our article we used a) the descriptive research based on the characteristics of the situations represented in the English novels: b) the componential analysis based on the characteristics of the idiom structure; c) the semantic analysis involving the description of the idiomatic meaning. In our article we have attempted to explore the variety of ways of the author's modification of idioms in the English literary discourse of the 19th centuries. We shall describe the cases of the idiomatic modification and analyze the specific features of some modified idioms in the prose of the following British writers: Ch. Dickens, L. Carroll, W.S. Maugham, A.A. Milne, O. Wilde. Having studied the most famous their novels we distinguished 5 modified idioms and analyzed the ways of their semantic and stylistic changes of these stable expressions. A first we defined the term “idiom” based on the scientific works of Russian and foreign linguists. Then we dwelled on the role of idioms in creation of the author's picture of the world. Describing the idioms, we proved a great role of these idioms in literary discourse. First of all, they characterize the literary personages and make their speech more emotional and expressive. Besides, some modified idioms create humorous effect and involve the reader in so-called wordpay. The authors often create puns based on idiomatic meaning. To prove it we analyzed some fragments of the novels written by the British authors.