It also said it is still legal for Underwater World Singapore to keep the highly endangered creatures, which are also known as Indo-Pacific humpbacked dolphins.

Still, the Animal Concerns Research and Education Society (Acres) said it will not give up its fight and has asked the secretariat of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (Cites), an international conservation agreement, to take up the case.

Underwater World, owned by Haw Par Corporation, set up the $3-million Dolphin Lagoon in 1999 with six pink dolphins it had acquired from a Thai marine park.

Acres said that when the park was raided last December, documents found showed it got its first dolphins in 1988, 16 years ago.

Since at least four of the dolphins here are more than 16 years old, this means they weren't bred in captivity, said Acres president Louis Ng. One of the four has since died.

He added that this means the convention has been breached, because it disallows the export of animals when this will affect their survival in the wild.

In a statement yesterday, the AVA blamed the mistake on a clerical error and said the Cites convention has not been contravened because the dolphins were brought in for educational and breeding purposes, not trading. It also said the park has other wild specimens of endangered animals on display, such as marine turtles, and there is no ban on the export of wild dolphins.

The AVA will liaise with its Thai counterparts to correct their records, but will not repatriate the dolphins.

Undeterred, Mr Ng has written to the Cites secretariat in Switzerland to ask it to investigate the case.

Haw Par Corporation and Underwater World declined to comment.