Police in Germany and Portugal have clashed over sharing DNA evidence that could blow open the Madeleine McCann case.
According to German newspapers, German investigators have requested a re-test of a mystery saliva sample that was found in the holiday apartment McCann vanished from in 2007.
German police say they know paedophile Christian Brueckner killed McCann, but need further evidence before they can charge him with her murder.
However, their relationship with Portuguese police has hit a bump, with the Portuguese "refusing" to give up DNA samples to the "arrogant" German police.
A source close to Policia Judiciaria labelled the re-testing "a total waste of time".
Portuguese police sources branded the German request "typically arrogant", adding that they had already carried out forensic tests on the sample and it was impossible to extract a DNA profile to compare against that of convicted paedophile and rapist Brueckner.
"We have already tried and it's insulting to be asked to do this again," a source close to the Policia Judiciaria said.
"This is a total waste of time. What's even worse is they now want to do it in their labs, as if ours aren't good enough. Why do they think their DNA testing centre is any better than ours?"
Peter Bleksley, a founder member of Scotland Yard's undercover unit, said: "DNA science has galloped on a long way since 13 years ago when Madeleine first went missing, so there are far more complex tests that can be applied now but couldn't have been done back in the day."
If successful, forensic experts would then try to match it with the DNA of German suspect Christian Brueckner.
Prosecutors in Lisbon previously turned down a Scotland Yard request to re-test samples in 2012 and could refuse the German authorities too.
More than 600 samples including hair and saliva were taken from the McCann's ground floor apartment in Praia da Luz in 2007.
They all remain under lock and key in the Coimbra lab but it is not known if Brueckner's DNA has been cross-checked with all the evidence.