Hong Kong's ambitious plans to expand its airport to meet soaring demand have sparked protests from environmentalists who say it would further endanger the city's rare Chinese white dolphins.

The southern Chinese city started a three-month consultation on its 20-year airport development blueprint last week, which includes a proposal for a new third runway due to booming cargo and travel demand in the region.

Airline groups have pushed for the third runway, which would cost up to HK$136.2 billion ($17.5 billion), to ensure the airport -- the world's biggest cargo hub in 2010 -- stays competitive on the global stage.

The project will be the city's costliest ever infrastructure project, taking into account expected inflation over the 10-year construction period.

But environmentalists say the project, which would include reclamation of 650 hectares (1,600 acres) of land from the sea, would threaten the survival of Chinese white dolphins, which are already facing population decline.

"The third runway is going to bring a huge problem to the white dolphins," Samuel Hung, Hong Kong Dolphin Conservation Society chairman, told AFP.

"It will be right at the centre of the dolphin population range in Hong Kong. (The affected area) is usually used as a corridor for them to travel back and forth. It will take the habitat away from the dolphins," he said.

Experts say there are about 2,500 of the mammals, also known as pink dolphins, in the Pearl River Delta region, the body of water between Macau and Hong Kong. About 100 are in Hong Kong waters with the rest in Chinese waters.

The dolphins, a sub-species of the Indo-Pacific humpback dolphins, are unique for their pink skin. They are listed as "near threatened" by the International Union for Conservation of Nature.

The mammal was the official mascot at the handover ceremony when the former British colony returned to Chinese rule in 1997, while dolphin watching is one of the tourist attractions in Hong Kong.

But Hung said their population has been in "significant decline" over the past few years, threatened by overfishing, an increase in maritime traffic, water pollution, habitat loss and coastal development.

"Hong Kong is blessed with the white dolphins despite such a small area of waters. It is very important for us to protect this population," Andy Cornish, director of conservation group WWF Hong Kong, told AFP.

"The environmental impact is going to be major. WWF is not anti-development but Hong Kong people need to be aware of the impact," Cornish said, referring to the airport expansion plans.

There are also fears that a third runway would worsen the city's already severe air pollution and hamper its target to cut greenhouse gas emissions by up to 33 percent by 2020, based on 2005 levels.

Poor air quality is a frequent complaint among the seven million residents of the densely populated financial hub, whose stunning skyline is often obscured by smog.

Advocates of a new runway, including home carrier Cathay Pacific and the International Air Transport Association (IATA), say a third runway is crucial, with the current two runways forecast to reach saturation point by around 2020, according to the airport authority.

The airport blueprint also includes another option, which is to maintain two runways and enhance facilities at an estimated cost of HK$42.5 billion.

"If Hong Kong International Airport does not expand, or fails to expand in a timely manner, to meet our future aviation traffic demand, there will be adverse consequences," Stanley Hui, Hong Kong's Airport Authority chief has warned.

The price tag for the new runway would be far higher than the HK$55 billion cost of the existing facilities at the airport, which opened in 1998, due to soaring construction material prices and the amount of reclamation required.

The airport, ranked third worldwide based on international passengers flown in 2010 after London and Paris, saw its busiest single day in April with 1,003 flight movements.

It handled 4.1 million tons of cargo and 50.9 million passengers in 2010.