Timmy the humpback whale is most likely dead, according to experts, just days after private donors spent £1.3million funding the controversial rescue.
The 33ft mammal - whose health had severely deteriorated after being stranded since March - was released on Saturday after being transported in a water-filled barge by rescuers.
But just days after Timmy entered the North Sea off Denmark, and following a lack of tracking data, the German Oceanographic Museum has determined he didn't survive the transition to deep water.
'Since the whale was in an extremely weakened state and repeatedly stranded itself within a short period of time after previous rescue attempts, it is highly likely that it did not have enough strength to swim in deep water for any length of time and is no longer alive,' the museum told the German Ostsee-Zeitung newspaper on Tuesday.
Scientists have long disagreed with the mission to save Timmy, insisting he was unlikely to survive rescue attempts due to his health and the more ethical approach would be to let him die peacefully.
But a motley crew of veterinarians and rescuers, spurred on by the national frenzy in Germany to save the whale, wouldn't take no for an answer despite fierce warnings.
The rescue effort was privately funded by two millionaires, MediaMarkt co-founder Walter Gunz and horse racing entrepreneur Karin Walter-Mommert, who said they were prepared to pay 'whatever it costs' to release Timmy.
The creature has been described as lethargic and covered in blister-like blemishes, and parts of his mouth were believed to be caught in a fishing net.
The stranded humpback whale was freed on Saturday following a rescue mission that cost £1.3 million
Timmy was transported in a flooded cargo ship after being recovered from a shallow bay off Wismar near the city of Lübeck
The creature first became stranded on a sandbank in Wismar Bay near the city of Lübeck nearly six weeks ago.
As his health deteriorated, German authorities abandoned attempts to save the mammal, insisting he could not be freed.
But following a spike in national interest surrounding Timmy - with supporters baking whale-shaped cakes and having themselves tattooed with images of the mammal - officials were persuaded to approve the privately-funded operation.
Initial attempts to save Timmy, involving inflatable cushions and pontoons, were unsuccessful, but last week divers eventually managed to coax the whale onto a flooded barge, towed by the Fortuna B ship.
On Saturday, the whale left the barge in the North Sea at around 8.45am local time, and was later observed blowing through its blowhole and swimming freely 'in the right direction', according to Walter-Mommert from the rescue mission.
Till Backhaus, the Social Democratic (SPD) environment minister for the state of Mecklenburg-Vorpommern, expressed hope for a 'happy end'.
And hope was still alive on Monday when the whale's GPS tracker apparently sent several signals in the morning, indicating that Timmy had surfaced from the water to breathe.
But now experts believe the GPS transmitter was faulty, lacking the necessary capabilities to detect signs of life, and Timmy's fate seems far less certain.
'I believe the whale will die very soon now. And I would also like to raise the question: What is actually so bad about that?' said Thilo Maack, a marine biologist at Greenpeace, earlier this month
Attempts to save the mammal had been criticised by the International Whaling Commission (IWC) as 'inadvisable', with experts saying the creature 'appeared to be severely compromised' and was 'unlikely to survive' attempts to move it into deeper water.
Continuing to try to save the creature amounted to 'pure animal cruelty', according to the director of the Oceanographic Museum in Stralsund, Burkard Baschek.
'A rescue attempt … is no longer worthwhile … this has been confirmed to us repeatedly by international colleagues,' he warned prior to the mission.
Experts from the museum warned again last Wednesday against letting the whale loose in the open sea, saying it was in danger of drowning, and called on the team behind the rescue operation to be transparent, including providing data on the mammal's whereabouts and publish its location if it was freed.
'I believe the whale will die very soon now. And I would also like to raise the question: What is actually so bad about that?' said Thilo Maack, a marine biologist at Greenpeace, earlier this month.
'Yes, animals live, animals die. This animal is really, really, very, very, very sick. And it has decided to seek rest.'
The Danish environment ministry has told the German TV broadcaster Deutsche Welle it was not planning to rescue Timmy if he becomes stranded again, describing whale beaching as a 'completely natural phenomenon'.