It has taken three decades, but a campaign to free wild animals including a female gorilla from the rooftop of an old Bangkok mall is gaining momentum in Thailand.

The 26-year-old female gorilla, named Bua Noi, has called a concrete enclosure on the mall's seventh floor home for more than 20 years.

It is the prized possession of the zoo's owner, Kanit Sermsirimongkol, who said he opened the zoo 30 years ago because he loves animals.

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"There is no regulation how big the cage should be for a monkey, a snake or a bird. There is no rules yet," Mr Sermsirimongkol said.

But Thai authorities now acknowledge that the high-rise zoo breaches a raft of guidelines and have ordered the gorilla and other large animals to be removed.

They said the animal must be removed by July, but admit it could be a difficult ruling to enforce.

Hundreds of animals, from orangutans to a black panther, a cheetah, a baboon and bears, are all squeezed into the one shopping mall floor.

Mr Sermsirimongkol said the animals were all well cared for.

But Bua Noi's one-time silverback mate, which had shared the same concrete enclosure, died a decade ago, leaving the social animal alone on a mall rooftop and animal welfare groups appalled.

Sinjira Apaithan has led a campaign to have the animal moved.

"I just felt so absolutely saddened by what I saw. We should have stopped this issue ages ago," he said.

"It's not the place for animals. So the main thing is education for Thai people, for Thai society, for a new generation to learn. This is the lesson that we have to learn it."

But Adisorn Noochdumrong, the inspector-general of Thailand's national parks and wildlife department, said moving the animal would pose challenges.

"Moving the big animals, especially the gorilla, is quite sensitive," he said.

"It is easy for her to get infection if she is moved to the ground and she could die. So it needs more study."

The animals' vet says he hopes another gorilla can be found and brought to Bangkok, so Bua Noi no longer has to live alone.