The German TV station behind an explosive documentary into the disappearance of three-year-old Madeleine McCann has revealed the decision that tipped off Christian Brueckner that police were investigating him.

McCann disappeared from a holiday apartment in Praia da Luz, Portugal but her twin siblings remained.

Prosecutors are currently investigating German national Christian Brueckner, a 43-year-old convicted sex offender, over the disappearance.

Germany's Spiegel TV claims that Brueckner was notified by letter that authorities were investigating him as part of the McCann case in November 2013, six years after her disappearance.

In internal documents cited by Spiegel, the letter was a single sheet titled "Subpoena as a witness" and instructed Brueckner to visit a police station for interrogation.

Police experts said that this decision was in no way a usual procedure to address a suspect head-on in a "delicate case", and Spiegel claims that Brueckner would have had ample time to remove evidence after being notified.

Portuguese detectives have already faced fresh scrutiny over the way they handled the case after missing leads and chances to secure potentially key forensic evidence.

The force confirmed that Brueckner was known to Scotland Yard's investigation but did not become an official suspect until 2017. This was due to information received after the 10th-anniversary appeal for information and subsequent inquiries.