Three-year-old Madeleine McCann vanished from a holiday apartment in Praia da Luz, in Portugal’s Algarve region, in 2007.
It has become one of the highest-profile, unsolved missing person cases in the world.
Brückner was not identified as a suspect in her disappearance until 2020.
German investigators classed the case as a murder inquiry.
However, no charges have ever been brought against Brückner, in the McCann case, and the full details of the alleged evidence have never been released.
A convicted child sex offender, Brückner is a German national who has a history of sex, forgery, drug, and theft offences.
A drifter, he lived in Portugal’s Algarve region, on and off, for years.
Now prosecutors are working against the clock as Brückner is scheduled to walk free in September 2025.
Investigators fear that Brückner, as a free man, will skip Germany and disappear.
A ruling, last year, paved the way for his release when he was acquitted of five other sex offence allegations.
In October 2024, judges cleared Brückner of three counts of rape and two counts of child sex abuse – charges dating back to between 2000 and 2017 in Portugal.
Prosecutors have launched an appeal, but a decision on any potential retrial may yet take months.
Brückner is due in court on Thursday morning in Lehrte, Lower Saxony, to face the charge of insulting a prison staff member, a court official told the BBC.
If found guilty, Brückner could face either a fine or up to an additional year in jail.
Interest in Brückner’s legal battles continues to be high due to his name being linked, by the authorities, to the disappearance of Madeleine McCann.
Prosecutor Hans Christian Wolters told the BBC he was aware of the upcoming, unrelated trial at Lehrte District Court, but that it is being handled by a different office.
He also reiterated there are still no plans to charge Brückner over the McCann case.
In 2020, Mr Wolters told the BBC the public would reach the same conclusion as his team had, about Brückner’s alleged involvement, if they “knew the evidence we had”.
The BBC has approached Christian Brückner’s legal team for comment.
Madeleine$ McCann# suspect faces new #unrelated charge