The Fault In Our Stars author John Green has issued an apology to a young fan after taking credit for a quote he thought he had written.

Fans have long attributed the phrase, "I'm in love with cities I've never been to and people I've never met", to Green's 2008 book Paper Towns, so he became convinced he had penned the thought-provoking line too.

He even sold posters featuring the words, but Green has since discovered the truth about the quote, which was originally sourced from fan Melody Truong's social networking page on Tumblr.com.

Truong was accused of stealing the line from Green, but when her friend went public with the story on Reddit.com recently it caught the author's attention.

In a YouTube.com video apology issued on Tuesday (17Feb15), Green reveals he actually illegally downloaded a digital copy of Paper Towns to search through the copy and check the story - and now he admits, "I'm the thief!"

He explains, "When I saw the quote, I assumed it came from Paper Towns. People kept sourcing it as coming from Paper Towns."

Now Green has started retroactively paying royalties from poster sales to Truong and he admits the Internet has made it easy for mistakes like this to happen.

He says, "We do an epically bad job of acknowledging one another's work and checking our sources."

A stunned Truong has since responded to the news, saying, "I'm still in a state of shock about the situation, actually. I never expected any of this to happen. I've always looked up to John Green as a role model. I know that if this happened to me with another person or company, nothing of this calibre would have occurred. He is the prime example of a wonderful person."

Paper Towns is set to be adapted for the big screen with British model/actress Cara Delevingne in the lead role.