The whale, first spotted in the River Seine last week, is dangerously thin and on the brink of death.
Environmentalists will attempt to save the life of a dangerously thin beluga whale that is stranded in the Seine River west of Paris.
They will on Tuesday try and move the four-metre mammal to a nearby saltwater basin so that it can be given urgent medical care.
If the whale does not die during the transfer -- and then responds well to treatment -- it could be released back into the open sea.
Conservationists were hoping to spare it the fate of another, an orca, that strayed into and then died in the Seine in May.
Sea Shepherd France (SSF) described its plans to move the beluga -- which is native to the icy waters around the Arctic -- as an “enormous operation”.
While the mission carries serious risks for the whale because of stress, it will not survive much longer in the Seine’s fresh water, added SSF.
French media said rescuers may use a barge and then a truck to relocate the whale, yet they have not yet revealed their tactics explicitly.
The beluga is currently in a section of the Seine river in Notre Dame de la Garenne, west of the French capital.
Due to the extreme heat in the region, rescuers will wait until temperatures get cooler in the afternoon before attempting to move the sea creature, which weighs 800 kilograms.
The whale is thought by the French authorities to be suffering from illness.
Hopes were raised for the whale's survival after it responded to a cocktail of antibiotics and vitamins over the last few days.
It “rubbed itself on the lock’s wall and got rid of patches that had appeared on its back," according to Lamya Essemlali, president of SSF.
Essemlali added that checks in the saltwater recovery basin will help establish whether the whale has "something we can help it with, or from an incurable illness.”
Rescuers have tried without much luck to feed the creature fresh fish since Friday, with Sea Shepherd fearing the whale is slowly wasting away underwater.
Authorities in the Eure region, where the mammal is currently stuck, said in a statement on Friday night that the beluga has “fleeing behaviour vis-a-vis the boats” and is ignoring their efforts to take it to safety.
Those attempting to help the whale have -- until today -- tried to leave the whale alone and “avoid stress that could aggravate its state of health,” according to a statement.