In an interview with Stern magazine, senior public prosecutor Helmut Walch said Mannichl had been “not consistent during his interrogations.”

The attack, which took place just over a year ago in the Bavarian city, left the then police chief in hospital with a serious stab wound. He said he had been stabbed by a tall skinhead but doubts have since been cast on the story.

Now, for the first time, the public prosecutor is openly expressing concerns, saying Mannichl’s evidence had been mired in inconsistencies.

For instance, Mannichl initially claimed there had been no prolonged contact with the attacker, but subsequently described the attack as “considerably more intensive,” Walch said.

“There are contradictions,” he said.

Walch also criticised Mannichl over the police chief’s complaint that investigators had not taken samples from under his fingernails immediately after the attack.

“I have to take the side of the police officers regarding the fingernail samples,” said Walch.

Mannichl was a “senior police officer and he knew about the power and the evidence value of fingernails,” he said.

Yet at no time had Mannichl asked the officers why they were not taking such samples.

“If he himself didn’t see it as necessary, then you can’t now go blaming the police officers.”

Mannichl was stabbed with a 12-centimetre knife and seriously injured on his own doorstep on December 13, 2008. He said at the time the attacker had told him: “Greetings from the national resistance.”

Earlier this month, prosecutors said there were still 430 open leads in the case and that 10 investigators were working on it. They had reviewed some 3,000 clues and interviewed 2,100 people but were yet to discover who the perpetrator might have been.