Only San Diego Zoo in the US and Adventure World in Wakayama, Japan, have four pandas. San Diego Zoo boasts a 15-year-old female, Bai Yun, the first giant panda to be successfully born at the Wolong Giant Panda Research Centre, and Gao Gao, a 14-year-old male. Their cubs, Su Lin and Mei Sheng, were born in the zoo. The adults are on loan from the mainland and the agreement expires next year.

Adventure World, which previously had six pandas, the most in the world outside the mainland, now has two adults on loan. Twin cubs were born last December. Cubs born overseas have to be returned to the mainland.

If Ocean Park's new additions have cubs in Hong Kong, they do not have to return to the mainland, deputy chief executive Matthias Li Shing-chung said.

The pair will be under quarantine for just over a month upon their arrival and live in separate quarters from An An and Jia Jia.

They will be ready for public viewing from July 1, the handover's 10th anniversary.

'They are special-meaning pandas,' chairman Allan Zeman said, referring to the handover gift.

A Wolong delegation visited the park in December and was impressed with An An and Jia Jia's condition given their age, which helped push the decision to grant the pair of cubs, Mr Zeman said.

Mr Li said he did not expect annual costs to care for the pandas to double from the roughly HK$8 million the park spends on An An and Jia Jia.

Much of the extra costs will come from the need for more bamboo and four more staff. The pair are expected to reach sexual maturity by the time the new enclosure, Astounding Asia, is completed at the end of next year.