A former leading European neo-Nazi has claimed that he was the middleman in the theft of the Auschwitz sign bearing the three most powerful words from the Holocaust: “Arbeit Macht Frei”.

According to Polish and Swedish investigators, the theft was organised by Anders Hoegstrom — who set up the virulently anti-immigrant National Socialist Front in Sweden in the 1990s. “My role was to get the sign in Poland,” he told the Swedish tabloid Expressen. “I was the middleman and was supposed to take care of the sale.”

Mr Hoegstrom claims that he turned himself in to the Swedish authorities when he suddenly became aware that the sign was to be sold to a collector and the money used to fund a campaign to disrupt the Swedish election campaign this year, if necessary with violence.

“That was not something I wanted to be involved in or carry out in any way,” he said.

The wrought-iron lettering — meaning Work Sets You Free — was nailed by the Nazis to the gate leading into the concentration camp where more than a million Jews were killed.

The sign was stolen last month by a gang of five Poles, apparently to be shipped to Sweden and then sold. It has since been recovered and the gang believed to be responsible is under arrest.

The Swedish intelligence agency SaePo has confirmed that it is investigating reports of a neo-Nazi plot to blow up the Swedish parliament, the Riksdag, and the home of the Prime Minister, Fredrik Reinfeldt. The leader of the Polish gang, named only as Marcin A by the Polish authorities, got to know Mr Hoegstrom two years ago while doing odd jobs on his family estate in southern Sweden.

When Mr A returned to his small building company in Poland, the two men stayed in touch.

Last autumn Mr A received word from Sweden that he should put together a group of experienced thieves to unscrew the sign and smuggle it out of the country.

The gang, aged between 25 and 39, with nicknames such as “Old Mouse” and “Sparrow”, realised that it was out of its depth when the theft on December 18 prompted an international outcry and a nationwide manhunt. The sign had been broken into three parts and hidden in woodland. Communication was reportedly then made between the Poles and Mr Hoegstrom, and the Swede decided to turn himself in to the police.

“I contacted the police immediately as soon as the sign was stolen and gave them all the information I had,” he said. “I haven’t committed any crime. I was the one who saw to it that the sign was found.”

The plot then thickens and indeed begins to resemble the bestselling thrillers of Stieg Larsson, who first made his name as a journalist investigating the far-right fringe in Swedish politics. Sweden’s neo-Nazi movement has shown itself capable of violence in the past. In 1999, for example, far-right attackers shot the trade unionist Bjoern Soederberg.

It was this and other attacks that spurred the journalistic endeavours of Mr Larsson — who worked for a while as the Scandinavian correspondent for the Britsh anti-fascist magazine Searchlight — and persuaded Mr Hoegstrom to change sides.

He renounced his party, which has since splintered and renamed itself, and now co-operates with a group that helps former neo-Nazis to return to normal life.

This leaves several questions unanswered. Why should Mr Hoegstrom be involved in such a high-profile and apparently ideologically driven theft if he has renounced his ways? Who was being lined up to be the final buyer of the sign? And who, if anybody, was really trying to raise cash to blow up parliament?