Despite scoring three bestsellers in five years and a clutch of awards, The Spinning Heart author Donal Ryan has been forced to return to his day job in the Irish civil service in order to pay his mortgage.

Ryan has become the latest casualty of tumbling incomes for writers. Despite receiving advances and signing a deal to write three more books with his publisher, the Irish novelist said he had found it impossible to earn a living wage as a full-time writer. “You need to have something else on the go,” he told the Irish Sunday Independent. “You could take a chance and scrape a living through bursaries and writing books, but I’d get too stressed out. It just isn’t worth it. I have two kids in school and I have a mortgage to pay.”

Saying his earnings amounted to about 40 cents per copy sold, he told the newspaper he had taken a job in the Workplace Relations Commission. “I would need to sell a huge amount of books to make a good salary out of [my books],” he said. He added: “I have to look at the long term and the fact that I have 20 more years of a mortgage.”

Ryan’s decision came as children’s authors hit out at celebrity children’s books. Tales of Terror author Chris Priestley told the Bookseller(£) that professional authors were finding it hard to compete for advances and shelf space. “It’s a tricky time in publishing at the moment,” he said. “I met a lot of writers last year who were having a hard time and in negotiations they were finding it harder to get the advances they got a couple of years ago.”

Priestley said that while the market was tough for all writers, celebrities were at an advantage competing for book deals. “It seems as though if you’re a celebrity you can just express the idea you would like to do a book – like [radio DJ] Christian O’Connell did on Twitter – and you will get a deal. I still have to pitch my books.”

In the last two years comedians and YouTubers have rushed into the market, some signing six-figure deals, while professional authors’ advances slipped to as low as three and four figures. Adding “children’s author” to their CV are the likes of David Walliams, Russell Brand, Danny Baker, Frank Lampard and Pharrell Williams.

CJ Daugherty, who writes thrillers for young adults, claimed ghostwritten children’s books risked undermining readers’ trust. “We can tell ourselves that readers must know a C-List celebrity, famous for opening makeup boxes on YouTube, isn’t capable of writing an 80,000-word novel,” she told the Bookseller. “But the whole system seems designed to fool people into thinking they are.”

Author and children’s book critic Amanda Craig told the trade magazine: “It’s distasteful [that] celebrities and their agents seem to think publishing a novel is a way to use their brand to make more money and, with the exception of David Walliams, they’re not very good.”

Authors’ incomes have fallen dramatically over the past 15 years, with many forced to give up writing full-time and find alternative employment to prop up earnings. According to data published by Queen Mary University for the Authors Licensing and Collecting Agency in 2014, only 10% of professional authors make £60,000 or more a year from writing, while the bottom 50% of authors earn less than £10,500. The bottom 50% of authors account for only 7% of the income earned by all writers put together.

While many authors have moved into teaching creative writing courses in tertiary education to shore up their income, others, such as Dan Rhodes, have been forced to take jobs outside the literary world; the Timoleon Vieta Come Home author now works as a postman.

On hearing Ryan’s news, author Liz Nugent tweeted: “Maybe now people will stop asking me why I’m driving a 13 yr old car.”