Ocean explorer Jean-Michel Cousteau has joined Peta in putting pressure on Thomas Cook to cut ties with SeaWorld.

The renowned explorer and environmentalist sent the travel company a letter this morning to coincide with its announced audit of the US theme park chain.

The letter read: "As someone who has had the great privilege of observing orcas in the ocean – where they belong – I felt compelled to contact you ahead of this inspection.

"We know so much more about wild animals today than we did decades ago when humans first started removing them from their families and forcing them to entertain us in captivity."

Costeau goes on to explain how the orcas' intelligence, culture and ability to feel a wide variety of emotions make them especially vulnerable to psychological and physical suffering in captivity.

Orcas also navigate by echolocation in the wild, but their sonar reverberates off the tank walls in marine parks such as SeaWorld, resulting in them being overwhelmed with their own vocalisations.

"If we accept, as we must, that orcas are highly intelligent, social animals who feel pain and joy, love and grief, and fear and longing, then we must also reject facilities that profit from denying these ocean dwellers everything that's natural and important to them," Cousteau concludes. "As such, the only logical, ethical outcome of this audit must be for Thomas Cook to sever its ties with SeaWorld entirely and immediately."

Cousteau joins Sharon Osbourne, Paul O'Grady, and more than 20,000 other people who have supported PETA – whose motto reads, in part, that "animals are not ours to use for entertainment" – in speaking out against Thomas Cook's continued sale of tickets to SeaWorld.

Due to the 2013 film Blackfish which exposed the treatment of killer whales in captivity, SeaWorld announced that attendance at the parks had dropped 5.2% from the previous year and profits had fallen 28% over that quarter. From 2014 to 2015, net income in the second quarter fell by 84%.