The German news magazine Der Spiegel has been plunged into chaos after revealing that one of its top reporters had falsified stories over several years.

The media world was stunned by the revelations that the award-winning journalist Claas Relotius had, according to the weekly, “made up stories and invented protagonists” in at least 14 out of 60 articles that appeared in its print and online editions, 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 won Germany’s Reporterpreis (Reporter of the Year) for his story about a young Syrian boy, which the jurors praised for its “lightness, poetry and relevance”. It has since emerged that all the sources for his reportage were at best hazy, and much of what he wrote was made up.

The falsification came to light after a colleague who worked with him on a story along the US-Mexican border raised suspicions about some of the details in Relotius’s reporting, having harboured doubts about him for some time.

The colleague, Juan Moreno, 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 the American football star Colin Kaepernick.

In a lengthy article, Spiegel, which sells about 725,000 print copies a week and has an online readership of more than 6.5 million, said it was “shocked” by the discovery and apologised to its readers and 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, which was founded in 1947 and is renowned for its in-depth investigative pieces, said Relotius had committed journalistic fraud “on a grand scale”. It described the episode as “a low point in Spiegel’s 70-year history”. An in-house commission has been set up to examine all of Relotius’ work for the weekly.

The reporter also wrote for a string of other well-known outlets, including the German newspapers taz, Welt and the Frankfurter Allgemeine’s Sunday edition. Die Welt tweeted on Wednesday: “He abused his talent”.

Relotius told Spiegel he regretted his actions and was deeply ashamed, the magazine said. “I am sick and I need to get help,” he was quoted as saying.

Moreno, who has worked for the magazine since 2007, risked his own job when he confronted other colleagues with his suspicions, many of whom did not want to believe him. “For three to four weeks Moreno went through hell because colleagues and those senior to him did not want to believe his accusations at first,” Der Spiegel wrote in an apology to its readers. For several weeks, the magazine said, Relotius was even considered to be the victim of a cunning plot by Moreno.

“Relotius cleverly rebuffed all the attacks, all of Moreno’s well-researched pieces of evidence … until there came a point when that didn’t work any more, until he finally couldn’t sleep any more, hunted by the fear of being discovered,” the magazine wrote.

Relotius, it added, finally gave himself up last week after being confronted by a senior editor.

In his confession to his employer, he said: “It wasn’t because of the next big thing. It was fear of failing. My pressure to not be able to fail got ever bigger the more successful I became.”

The magazine, which is one of Germany’s most prominent news organisations, is now trying to rescue its reputation amid fears a magazine already challenged by the problems in the German newspaper industry will struggle to recover.

“All [his] colleagues are deeply shattered,” the magazine wrote. In particular, it said, in the Society department, where he worked, “[his] colleagues are astounded and sad … the affair feels like a death in the family.”