The US government has waded into the scandal of the German journalist for Der Spiegel magazine who faked stories on a grand scale over years, calling it proof of “institutional bias” in the media against America.

In a scathing letter to the magazine’s editors, Richard Grenell, US ambassador to Germany, claims the journalism of Claas Relotius, who resigned from the German news magazine last week, was symptomatic of anti-American bias across the mainstream media. “It is clear that we were the victims of a campaign of institutional bias,” Grenell wrote to Der Spiegel, in a letter also seen by the daily newspaper Bild. He said he was aghast at the way “anti-American coverage” had been facilitated by the magazine.

Relotius had made many reporting trips to the US, and at least three of the resultant stories are now known to have been entirely or partly made up. They include an article published in March 2018 about a 59-year-old woman who travelled by bus around America to witness death row executions. It emerged last week that it had been completely fabricated.

Earlier this year Relotius interviewed 99-year-old Traute Laufrenz, the last survivor of the anti-Nazi White Rose resistance group, in Charleston, but at least parts of the text and details of when the interview took place are also believed to have been made up.

Der Spiegel yesterday issued an unprecedented mea culpa, printing a plain cover in its trademark orange, with the words: “Tell it like it is. On our own account: how one of our reporters falsified his stories and why he was able to do so.” Inside, over 23 pages, it details the drama surrounding the Relotius revelations, and praises his colleague Juan Moreno, who uncovered the scandal.

“This house is shocked,” it wrote in a leading article. “What’s happened to us is the worst thing that can happen to an editorial team.” It accused Relotius of “relying not on research but on his imagination”. In a 10-page essay entitled “A Nightmare”, the magazine offers an in-depth exposé of how the scandal came to light and the methods he used to cover up his tactics. Moreno, the journalist who revealed the scandal, is also given space to explain how he had come to the shocking conclusion that his colleague was fabricating material. Moreno started his own research against the will of Der Spiegel’s editors, and was almost fired over his repeated accusations, which his bosses at first did not want to believe.

The German media world is still reeling as a result of the exposure, which has been compared to the fraudulent Hitler diaries that were published by Stern magazine in the 1980s.Die Welt is one of several publications which published Relotius’s work before he became a Spiegel staffer, and is now examining the veracity of those articles. Christian Meier, its media editor, described as being reliant on “a group of reporters with a strong power base that have remodelled the magazine into a penmen’s journal, under which strong research has suffered”.

He added that questions were now being asked as to whether “the form of condensed, extremely dramaturgical edited reportage journalism that the Spiegel as well as other media practice is still sustainable”.

The scandal has sparked fears that the far right will exploit the scandal to sow further distrust of the media. The German far right has a long history of attacking the press.

In recent years, the anti-immigration group Pegida and elements of the Alternative for Germany (AfD) have resurrected the Nazi-era slur of Lügenpresse (“lying press”) to describe mainstream journalism they claim does not represent the world as they see it. These voices have been further emboldened by US President Donald Trump’s attacks on the media and his use of the term “fake news.”

“Relotius is in the end only a product of an absurdly leftist writers’ fraternity that is increasingly seldom prepared to leave its own convenient moral comfort zone in favour of the facts,” wrote Alice Weidl, a leader of the AfD, in a Facebook post.

The leading German journalist Hendrik Wieduwilt wrote: “It’s started! The fraud of ‘reporter’ Relotius has now been made into ‘fake news’, or strategically fraudulent lies. The AfD will exploit this for all it is worth. That’s probably the biggest damage of the whole scandal.” The independent media journalist Stefan Niggemeier took to Twitter to express fears the case represented a “deep blow – not just for Der Spiegel, but for German journalism.” In a series of soul-searching written apologies, the magazine acknowledged the wider undermining affect Relotius’s actions will have on those striving to deliver objective, informative and well-sourced reporting.

“We are aware that the Relotius case makes the fight against fake news that much more difficult,” wrote the incoming Spiegel editor-in-chief Steffen Klusmann and deputy editor-in-chief Dirk Kurbjuweit in a joint open letter to readers. “For everyone. For other media outlets that are on our side and for citizens and politicians who are interested in an accurate portrayal of reality.”