It's the case that has absorbed Scandinavia's elite artistic circles and tested some of Norway's finest literary experts.

Over the next few months, investigators from the Norwegian police's economic crimes unit will be combing the market for supposed possessions and letters relating to the playwright Henrik Ibsen, and the Nobel-winning novelist – and Nazi sympathiser – Knut Hamsun as part of investigations into an alleged scam that exploited the nation's interest in its most celebrated authors.

More than a dozen documents are alleged to have been forged by Geir Ove Kvalheim, a Norwegian scriptwriter and actor, who has been charged and is due go on trial in April.

The alleged fraud was only revealed when Kvalheim sensationally claimed to have discovered fragments of a previously unknown Ibsen play, The Sun God, a find that would have changed Norwegian literary history.

Lars Frode Larsen, a Hamsun expert who was one of the first to raise the alarm, said that he could not think of a literary forgery of such magnitude since the fake Hitler diaries in 1983.

"He was very convincing," Rolf Warendorf at Oslo's Norlis antiquarian booksellers, told the Observer. "His story was that he was a collector of all kinds of stuff connected to the second world war – uniforms, medals etc – and that he had got in touch with the older Nazis living in Spain and Norway."

To Warendorf's embarrassment, his bookshop became the conduit through which several of the alleged forgeries were brought to market. He bought a "signed" first edition of the Ibsen play John Gabriel Borkman, which the writer had dedicated to Edvard Munch, the artist who painted The Scream.

Warendorf also bought a pocket almanac from 1943, in which Knut Hamsun had apparently drafted a letter to Adolf Hitler, apologising for a tempestuous meeting in July that year. Eight of the documents were sufficiently credible to be bought by Norway's national library for a total of £75,000.

Jørgen Cappelen of Cappelens booksellers was another expert who aired his suspicions of Kvalheim. In 2006, he refused to buy the fragment of The Sun God, which Kvalheim had sent to him for scrutiny. "We didn't recognise the writing. We just didn't recognise the signature of Ibsen," he said.

After that, a group of antiquarian booksellers worked with the national library, bringing in the country's leading Ibsen and Hamsun experts to investigate the works. Initially Norwegian police refused to take up the case. It was only in 2008 that their economic crimes unit decided to investigate, bringing in experts who judged that at least 13 items, sold by Kvalheim for a total of £50,000, were forgeries, leading to Kvalheim's indictment at the end of November.

Warendorf believes that the true number of alleged forged documents could be much larger.

"He has been very active in the private market. My guess is this might be just a fraction of what he's sold," he said.

In the years running up to the case, Kvalheim had built up close links to Norwegians who had worked with the Nazis during the second world war.

Among those he befriended was Finn Thrana, a senior member of Norway's wartime fascist government, whom he claimed had given him the Hamsun almanac. He also befriended Fredrik Jensen, a former member of the Waffen-SS and the only Norwegian to be awarded the German Cross in Gold, one of the highest Nazi military decorations.

The relationship with Jensen broke down after Kvalheim passed the police footage of interviews he had carried out for a documentary, which he claimed proved that Jensen had helped shelter Nazi war criminals. In 2007, Norwegian courts ordered Kvalheim to pay Jensen £40,000 in damages for this claim, which resulted in Jensen being accused in newspapers of sheltering Aribert Heim – the Austrian nicknamed "Dr Death" for his gruesome medical experiments on inmates at the Mauthausen concentration camp – at his home in Málaga, Spain. But by the time Jensen died, aged 90, in July last year, it was believed that Heim had in fact died in Cairo in 1992.

After the 2007 case, Jensen went to the police accusing Kvalheim of forging a letter from him that gifted Kvalheim the documentation surrounding his award of the German Cross in Gold. That accusation is now one of the charges levied against Kvalheim by the prosecutors.

The alleged scam comes at a time of revived interest in Ibsen in the UK. In 2011, the National Theatre staged the UK premiere of Ibsen's Emperor and Galilean, London's Arcola Theatre revived his classic A Doll's House, and the Jermyn Street Theatre revived Little Eyolf.

Ibsen was championed by writers such as George Bernard Shaw and Thomas Hardy, who campaigned for the single London performance of his play Ghosts in 1891. The play, which the Daily Telegraph described as "a loathsome sore unbandaged", "a dirty act done publicly" and "literary carrion", had been banned by the censors because of its depiction of marital infidelity and venereal disease.

Prosecutor Aud Slettemoen said that the trial could continue throughout the year if Kvalheim appealed. "This could be quite a complicated case. We've never had a case like this before," she said.

Kvalheim, who failed to respond to a Norwegian newspaper's requests to comment on the accusations, faces up to six years in prison if found guilty.