By Alex Kirby BBC News Online environment correspondent

Conservationists are joining a European aquarium chain to campaign for a worldwide ban on the cutting-off of sharks' fins.

They say the practice is severely damaging many shark species, with implications for the marine environment.

The Shark Trust and the European network of Sealife Centres are launching a joint campaign for a ban.

They hope to collect a quarter of a million signatures in seven countries.

The campaign starts with a pair of divers plunging into a tank of sharks at the Sealife Centre in Birmingham, UK, on 30 April.

Lethal mutilation

The divers will put the first names on a petition that is to be presented to the United Nations later this year. All the sharks are British species, and none is dangerous.

Blue sharks in Vigo fish market, Spain (J Nightingale)

The campaigners say humans kill 100 million sharks every year, many of them purely for use in shark-fin soup.

The creatures' fins are often cut off while they are still alive, with the sharks returned to the sea to be eaten by other sharks or to die slowly.

Sharks are key predators at the top of the food chain. As their numbers decline, other species expand to occupy the vacant niche, changing the balance of marine life.

Large supplier

The campaigners say some restaurants sell shark fin soup for up to �70 ($100) a bowl, with a single dorsal fin from a whale shark or a basking shark fetching more than �10,000 ($14,500).

Fins for sale in Vigo (J Nightingale)

The whale shark is the largest fish in the sea. Basking sharks are found in UK waters, as are blue sharks, which the campaigners say are now increasingly rare summer visitors to the seas off south-west Britain.

They say at least 27% of fins imported into Hong Kong come from Europe. European exports in 1999 totalled two million tonnes of fins.

Dumped at sea

Shark Trust supporters who visited the Spanish port of Vigo in 2000 estimated the number of sharks landed there daily at 4,000.

They say this figure did not include sharks finned at sea, whose bodies were not brought ashore.

The campaign is also supported by Greenpeace-Germany and the International Fund for Animal Welfare.