London, Feb. 4: Coral reefs could be dead within two generations and cod replaced by jellyfish because of the acidification of the sea, scientists said yesterday.

The potentially disastrous problem, discovered only recently, is being caused by the build-up of carbon dioxide in the atmosphere.

It is parallel to man-made climate change and scientists believe that it will give new urgency to efforts to phase out fossil fuels. Carol Turley, the head of science at the Plymouth Marine Laboratory, told a conference in Exeter that the acidity of the sea was rising through chemical processes that turned carbon dioxide into carbonic acid.

She said: 'It is happening now; nobody is saying it is not happening. It is -level chemistry but no one noticed until 15 months ago. This is a rapid change that the world ' and the organisms in the sea ' have not seen for hundreds of thousands of years, if not millions. There is a very urgent need to do more research.'

Corals' ability to produce calcium is expected to decline by up to 40 per cent by 2065.

Lobsters and crabs, which form their shells out of another compound known as chitin, may be less susceptible to damage.