It is one of Britain's most threatened or fastest declining species and habitats, for which rescue plans have been proposed by a steering group of government scientists and wildlife conservation groups.
The best-documented threat it faces is from bottom-set drift nets, which are like curtains, several miles across, running along the sea-bed. The porpoises become entangled in these nets and drown; surveys have suggested about 10,000 die this way each year, mostly in the North Sea, off the coast of Denmark, and the Celtic Shelf waters off south-west England and southern Ireland.
But scientists believe the porpoise, phocoena phocoena, is also vulnerable to long- lasting, toxic pollutants which flow down rivers into the sea, get into the small fish it preys upon and then accumulate in the porpoise's body fat. It may also be frightened away from busy areas by the noise and movements of ships and boats.
The males grow up to 1.7m long. The females are sexually mature at only 14-months-old and they give birth to single calves. They are shy and secretive, compared to dolphins, which are often bold, curious and playful with people and ships. Consequently, little is known about the porpoise's social and family life. It surfaces only briefly to breathe. Earlier this year it emerged that dolphins sometimes kill their smaller relatives in British waters, ramming and battering them.
In 1994, Britain's Sea Mammal Research Unit in Cambridge organised a large survey of cetaceans in the North Sea, the English Channel and Celtic Shelf, using ships and aircraft and funded by the European Commission. This led to the first population estimate for the porpoise - between 267,000 and 465,000 dwell in these seas. The Steering Group proposes a target of maintaining this population and ensuring in the long term that no man-made factors stop the porpoise returning to waters where it was formerly found.
Among the recommendations for achieving this are further river and coastal pollution curbs, and changes in fishing practices and net design to reduce the drownings.
The cost is estimated at pounds 250,000 a year.