Claas Relotius was named German Reporter of the Year for a story about a young Syrian boy this month

Germany's respected news weekly Der Spiegel stunned the media world on Wednesday by revealing that one of its award-winning reporters had for years falsified stories.

Claas Relotius "made up stories and invented protagonists" in at least 14 out 60 articles that appeared in the magazine's print and online editions, Spiegel wrote, warning that other outlets could also be affected.

Relotius, 33, resigned after admitting to the scam. He had written for the magazine for seven years and won numerous awards for his investigative journalism, including CNN Journalist of the Year in 2014.

Earlier this month, he was named German Reporter of the Year for a story about a young Syrian boy.

The cheating came to light after a colleague who worked with him on story along the US-Mexican border raised suspicions about some of the details in Relotius's reporting.

The reporter eventually tracked down two alleged sources quoted extensively by Relotius in the article, which was published in November. Both said they had never met Relotius.

Relotius had also lied about seeing a hand-painted sign that read "Mexicans Keep Out", a subsequent investigation found.

Other fraudulent stories included one about a Yemeni prisoner in Guantanamo Bay, and one about NFL star Colin Kaepernick.

In a lengthy article, Spiegel said it was "shocked" by the discovery and apologised to its readers as well as to anyone who may have been the subject of "fraudulent quotes, made up personal details or invented scenes at fictitious places".

The Hamburg-based magazine, renowned for its in-depth investigative pieces, described the episode as "a low point in Spiegel's 70-year history".

An in-house commission has been set up to pore over all of Relotius' work for the weekly.

The reporter also wrote for a string of other well-known outlets, including German newspapers taz, Welt and the Frankfurter Allgemeine's Sunday edition.