Sky News reports that Christian is now appealing his extradition arrest warrant under claims that it is not valid.
The rape occurred in 2005, two years earlier than Madeleine disappeared whereas holidaying along with her household in the identical resort.
The 43-year-old German was extradited to Germany from Italy under the warrant in 2018 and subsequently convicted and sentenced for the 2005 rape of a 72-year-old American woman in Portugal in December 2019.
He was living nearby at the time of Madeleine's disappearance and telephone data suggests he was near the apartment an hour before she was last seen.
He argued that he had been extradited from Portugal to Germany in 2017 on the basis of a drugs charge, not the rape, making the case against him invalid.
Thursday's announcement was simply an opinion and not legally binding, but the court tends to follow the advice of an advocate general.
Brueckner's legal challenge centres on whether the German authorities needed Portugal's consent to bring rape proceedings because of the 2017 extradition.
"Finally, in 2018, Germany obtained Italy's agreement that the accused will also be prosecuted and tried for rape and extortion".
CJEU Advocate General Michal Bobek affirmed the procedures for extraditions.
It comes ahead of a final decision by ECJ judges at a later date.
If that appeal is successful, Christian B could go free next year, at the end of his jail term for the drug offence.
Madeleine McCann murder suspect Christian B. was arrested in Italy before being handed over to German authorities.
Christian B was revealed as the main suspect in the case in June, as German and United Kingdom police made a fresh appeal for information about Madeleine's disappearance.
German prosecutors said in June they had "concrete evidence" that Madeleine was dead, despite British police continuing to treat her disappearance as a missing persons case.
After the appeal he said: "Ultimately, we can not influence the decision because it is not related to the McCann investigation".
"As long as he is in custody, we know this".
Last month, the Metropolitan Police said the case remained a "missing persons" investigation in the United Kingdom because there was no "definitive evidence" as to whether Madeleine was alive or not.
On July 29 German police completed their search of an allotment in connection with McCann's disappearance, but did not say what, if anything, they had found to resolve the years-old mystery.