Prosecutors fear the prime suspect in Madeleine McCann's disappearance could flee the country if he is released from a German prison on parole next week.
Christian Brueckner's lawyers have filed a request for him to be released after serving two thirds of his 21-month drug sentence in Kiel, Germany.
The case has now been passed to Germany's Federal Court in Karlsruhe and officials are expected to decide within the next week if he should be released.
In 2011, the 43-year-old was sentenced to 21 months in jail for his part in a drugs trafficking ring that supplied marijuana to VIP clients.
German prosecutors this month announced that the convicted paedophile and rapist was the prime suspect for 3-year-old Madeleine's 2007 abduction in Praia da Luz, Portugal.
Lead prosecutor Hans Christian Wolters said they had "concrete evidence" that the girl is dead.
A source close to the German investigation said: "If the superior court decides to free him then it will severely impact the case.
"He could vanish and then we will not be able to put him on trial. We are fighting for him to be kept in court."
The suspect's legal team have taken the case to the European Court of Justice claiming that there was a breach of European Arrest Warrant regulations when Brueckner was extradited from Italy to Germany.
He was flown to Germany in 2018 on a warrant issued for the drugs offence.
However, he was also then put on trial and convicted of the rape of a 72-year-old American woman at her villa in Praia da Luz in 2005.
In December, he was sentenced to seven years but is appealing the conviction. Under German law, the jail term has yet to be imposed.
Brueckner is a serial child abuser and a rapist, who was first convicted of sexually assaulting children.
Brueckner grew up in Wurzburg in Bavaria. He was just 15 when he had his first brush with the law, breaking into a house in 1992.
Two years later, aged just 17, he was convicted for the first time of child sex offences, including "performing sexual acts in front of a child". He was sentenced to two years but released partway through.
Brueckner decided he needed a new start in life so he moved to the town of Lagos with his girlfriend in 1995.
The town is just a few kilometres east along the coast from Praia da Luz.
He had planned to start his own business but in 1999 the authorities caught up with him again. Brueckner was arrested and extradited to Germany where he served a short sentence for further juvenile crimes.
On his release, he returned to Portugal, where he appeared to prosper. He rented a run-down farmhouse in Monte Judeu on a hillside inland between Praia da Luz and Lagos and made it his home.
In the early 2000s, Brueckner was enjoying a life of petty crime. Police now believe he was funding his lifestyle through breaking and entering and dealing cannabis. He was jailed for short stints in Portugal for stealing diesel and selling stolen passports and other property.
There are suggestions he had got himself caught up in a gang operating on the Algarve.