Case presented after American editor defends use of characters – because most stories have passed into public domain

The estate of Sir Arthur Conan Doyle has put forward a defence ingenious enough for Sherlock Holmes himself in a US copyright case that could redraw the boundaries of copyright law to recognise "complex literary characters".

The crux of the case lies in whether use of the characters Sherlock Holmes and Dr Watson is covered by copright law until the entire Holmes canon is out of copyright in the United States. At present, 10 stories from the final collection, The Case-Book of Sherlock Holmes, remain in copyright, with the stories due to enter the public domain in different years up to 2022.

Sherlockian editor and Los Angeles entertainment lawyer Leslie Klinger filed a suit in February with the aim of establishing that the characters of Holmes and Watson are already in the public domain in the US, after he was asked to pay for a licence to use them in his planned book In The Company of Sherlock Holmes. Klinger previously co-edited A Study in Sherlock: Stories Inspired by the Holmes Canon, which includes stories penned by writers Lee Child, Neil Gaiman and Michael Connelly.

In its defence, filed this week in Illinois district court, the Doyle estate argues that the characters remain protected until the copyrights in the final stories expire, because the subtleties and quirks of character that define the super-intelligent detective, his trusty right-hand man, and the duo's relationship, were developed throughout the entire body of works.

Jonny Lee Miller, right, as Sherlock Holmes and Lucy Liu as Watson in the television series Elementary, which sets the detectives in modern-day New York. The Conan Doyle defence contrasts the author's development of Holmes with 'flat, simplistic' TV characters. Photograph: CBS

"The facts are that Sir Arthur continued creating the characters in the copyrighted Ten Stories, adding significant aspects of each character's background, creating new history about the dynamics of their own relationship, changing Holmes's outlook on the world, and giving him new skills. And Sir Arthur did this in a non-linear way," the statement for the defence claims.

It continues: "Plaintiff [Klinger] suggests that Holmes and Watson can be dismantled into partial versions of themselves. But a complex literary personality can no more be unraveled without disintegration than a human personality."

Klinger has until 24 September to respond, according to his website Free Sherlock!

The argument goes on to claim that Holmes and Watson are unlike the "flat, simplistic entertainment characters" of television programmes, saying that "such characters genuinely are created in the first work in a series".

Conan Doyle wrote four novels and 56 short stories featuring Holmes. The adventures began with the novel A Study in Scarlet in 1887, and ended with the story collection at the heart of the case, The Case-Book of Sherlock Holmes, in 1927.

In the US, all works published before 1923 are in the public domain, and those published afterwards are protected by copyright for a maximum of 95 years after the death of the author. In the UK, copyright runs for the life of the author plus 70 years, meaning that all works written by Conan Doyle, who died in 1930, have been out of copyright for more than a decade.