They've all been carried across the world in the water used inside ships to provide stability during voyages and they now constitute one of the world's most serious environmental problems.

People never realised: it was such an obscure way of transporting potentially harmful creatures from one side of the globe to the other that nobody gave it any thought until it was too late. But ships' ballast water has been shown, time and again, to be the medium in which alien invasive species have gone from a homeland where they are benign to a new habitat where they cause environmental havoc.

This week in London, more than 100 countries, including Britain, are expected to sign a UN treaty regulating the management of ballast water by vessels around the world.

Its aim is to halt the spread of aquatic organisms, from jellyfish to crabs, from algae to mussels, which can be devastating in new ecosystems, when discharged with ballast water at a ship's destination.

Examples abound: the European zebra mussel is harmless on this side of the Atlantic, but transported in ballast tanks to the Great Lakes between Canada and the US, it causes ecological chaos, fouling underwater structures and pipes and resulting in pollution control costing of billions of dollars.

Or take the problem in geographical reverse: the comb jellyfish fits into the ecosystem of the US but ballast-transported to the Black Sea, it has depleted native plankton stocks so far as to cause the near-extinction of anchovy and sprat fisheries.

Algae that cause toxic algal blooms are known to have been transported and it is even suspected that epidemics of some diseases, such cholera, may be directly caused by ballast water when a new strain is transferred to a different part of the world.

Although the problem is little appreciated by the public, it is growing urgent as world trade continues to expand and has been identified as one of the four great threats to the health of the oceans (the others being land-based marine pollution, over-exploitation of fish and other natural resources, and direct destruction of marine habitat).

"It is an extremely serious environmental issue," said Efthimios Mitropoulos, secretary general of the International Maritime Organisation (IMO), the London- based UN agency under whose aegis the treaty is being negotiated.

"The fact of the matter is that ships, by carrying thousands of tons of ballast water from one part of the world to another, can transfer pathogens and other micro-organisms and invasive species that have the capacity to distort and destroy the delicate balance which exists in the ecosystem of the region where the water is offloaded."

Unlike oil spills and other marine pollution caused by shipping, Mr Mitropoulos added, exotic organisms and marine species could not be cleaned up or absorbed into the oceans. "Once introduced, they can be virtually impossible to eliminate and, in the meantime, may cause havoc."

Ballast is any material used to balance an object and ships have carried solid ballast, in the form of rocks or sand, for thousands of years. Modern metal vessels, however, use water, which is much easier to pump on and off. It is essential to provide balance and stability to ships when they are unladen.

A ship sailing with an empty hold will have filled its ballast tanks at its source port, and when it reaches its destination port and takes on cargo the ballast water will be discharged. With it may go any number of tiny living creatures picked up at the source port through the ballast water intakes.

It is not only a question of microbes and sm