When Nick Milligan decided to self-publish his speculative fiction novel, Enormity, he knew it was going to be a hard slog to find an audience. But seeing a similar plot play out in the trailer for Danny Boyle’s new film, Yesterday, came as a shock.

“I had high expectations for Enormity’s success,” Milligan said. “I wrote it with a movie in mind.”

That goal seems even more out of reach now.

Written by Richard Curtis and starring Himesh Patel and Lily James, Yesterday follows a character called Jack (Patel) who has a bicycle accident during a worldwide blackout. According to the trailer, which was released last week, when Jack wakes up, he finds himself in an almost identical version of Earth in which The Beatles never existed. He passes off their music as his own, and havoc ensues.

It’s a plot that bears remarkable resemblance to the Australian Milligan’s novel, which also follows a character called Jack who, after a journey into deep space, finds himself on a planet that’s almost identical to Earth, with a few exceptions – including that its people have never heard of The Beatles.

“He passes off classic music as his own material, including that of The Beatles, and the story then explores the consequences of that lie,” Milligan told Guardian Australia.

“The central premise and general exploration of the concept are the same. Both Jacks experience inner turmoil in regard to the lie they’re living and perpetuating,” Milligan said. “They’re both morality tales. Both are satires on the music industry. And the trajectory to superstardom, with Jack performing to a crowded stadium etc, appears to be in both.”

But there are differences in tone, Milligan points out.

“The tone of both, outside of the central premise, appears quite different. Yesterday is a more light-hearted family-friendly film, where Enormity is far more dark and twisted,” said Milligan. “It’s probably just a horrible coincidence and they mean me no disrespect.”

Milligan self-published Enormity in 2013, selling on Amazon. He estimates with giveaways and direct sales, he saw some 20,000 downloads of his book.

He chose to self-publish rather than trying to find a traditional publisher because he felt his idea “didn’t quite fit into a particular genre, so it might have been put in the too hard basket” by commercial publishers.

“Being an independent author, as I see it, is a long game. You have to be realistic. Rather than a big marketing budget you rely on word of mouth. It’s generally a slower process. Each new novel you write ultimately promotes the previous books. So my expectations have never been of overnight success,” Milligan said.

Research from Macquarie University in 2015 showed Australian authors earn only an average of $2,600 per annum from self-publishing genre fiction, the highest-earning area in self-publishing.

Since speaking out over the weekend about the similarities between Enormity and Yesterday, Milligan said he has seen a modest spike in sales of his own work.

“I honestly feel quite awkward about the whole situation. On one hand it’s amazing that Enormity is finally getting some exposure, but this is not the way I wanted it to happen,” he said.

“I hope people don’t read my book expecting it to be a light-hearted romp or a carbon copy of what’s in the trailer.”

Australian copyright law protects the expression of an idea but not the idea itself. According to the Australian Copyright Council, the basic plot for a book, television series, film or play, including the characters, is not protected by copyright, but the literary work itself is protected.

Grant McAvaney, CEO of the Australian Copyright Council, told Guardian Australia that to prove copyright infringement, one creator would need to prove that the other had taken a material portion of their work. “So that’s not just the idea alone, but it’s the way the idea is explored … by reference to things such as structure, characters, key plot points and language used,” he said.

But copyright law also varies across territories and is notoriously complicated. Even a claim pursued in Australia would result in a costly and time-consuming court case, said McAvaney. “Adding an international dimension adds to the cost and the complexity because the courts of different countries will take slightly different approaches to copyright infringement. Even if you win, enforcing that judgment against someone based overseas can be very difficult.”

“This is certainly not the first time a writer has felt this way – concerns were raised by the family of a later author about Shape of Water; and several authors felt that Avatar drew on their own creations,” McAvaney said.

Yesterday is due out in June. Milligan is understood to be seeking legal advice on his situation.

The producer of Yesterday, Working Title Films, has been approached for comment.