A memo to airline staff said the flight, which included a two-hour refuelling stop in Hong Kong, netted HK$850,000 in cargo revenue and that executives wanted to develop the business.

Since the memo was leaked on Wednesday, more than 2,400 people have signed an online petition protesting against the shipment and Hongkong Airlines' phone lines and website have been jammed with angry calls and messages.

A company spokeswoman said yesterday it was fully 'committed to the protection of animal welfare' and wanted an 'open dialogue' with animal welfare groups.

A copy of the petition, a DVD of The Cove and a protest letter was presented to an airlines official by the Sea Shepherd pressure group during a recruitment day for cabin crew at its Tung Chung headquarters yesterday.

The Whale and Dolphin Conservation Society (WDCS) and the Humane Society International (HSI) have written to Hongkong Airlines president Yang Jian Hong warning of a boycott unless it gives an undertaking not to transport dolphins.

'This shipment poses severe conservation and welfare concerns for our organisations and we urge your airline to refuse any further shipments of dolphins,' says the letters signed by WDCS campaigns manager Courtney Vail and HSI marine mammal scientist Naomi Rose.

Janet Walker, head of Hong Kong Dolphinwatch, said: 'Hongkong Airlines obviously has no idea how much vitriol is being stirred up by this and how badly it reflects on Hong Kong. We have had many messages of support saying Hong Kong shouldn't call itself Asia's World City if it can't meet world moral standards in animal welfare.'

Hongkong Airlines declined to answer questions about where the dolphins flown out of Japan originated from or reveal their final destination, which was believed to be an aquarium in northern Vietnam.

In an e-mail reply to questions from the Sunday Morning Post, an airline spokeswoman said: 'Please understand that from the company and commercial point of view, we cannot disclose the identity of the clients. But no dolphins suffered or died during this transportation.'

In a separate statement, the airline said the January 16 dolphin transfer 'fully complied with government rules and International Air Transport Association live animal regulations'. It added that import and export permits to meet the requirements of the Convention on International Trade in Endangered Species had been issued.

'Professional experts from Vietnam and Japan were sent there for evaluation before delivery,' the statement said. 'They advised that transport by air is the best solution for the transfer of live dolphins.

'Sea animal experts and veterinary surgeons were on board to supervise and monitor the transportation process and proper care was exercised during the entire voyage by the experts.'

A Hongkong Airlines spokeswoman said: 'Hongkong Airlines is fully committed to the protection of animal welfare. We appreciate the concerns raised recently by various animal welfare concern groups ... We will conduct open dialogue with environmental protection and animal welfare organisations.'

The company had previously said it would 'carefully examine the country of origin of any animal shipment and the purpose with potential clients before taking decisions'.

2,400

The number of signatures collected so far by an online petition protesting about the shipment of dolphins flown from Japan to Vietnam