Swimmers can enjoy most parts of the Danube and diners can savour its fish after a major cleanup, but Europe's second-longest river still suffers serious pollution near some major cities, according to a report.

Flowing some 2,850 kilometres from Germany's Black Forest to the Black Sea off Romania, the Danube has shown good overall improvement since 2001, said the report by the International Commission for the Protection of the Danube River (ICPDR).

It said plants were flourishing in previously polluted flood plains near Vienna and in the Danube delta, while swimmers could take to its waters between major cities. But dirty waters still prevailed in Budapest, Belgrade and Bucharest. "It's best to swim away from the big centres with over one million people," said Igor Liskea, who managed the ICPDR report.

"In Vienna, you wouldn't die if you swam in it, you wouldn't get very ill, but it's not advisable." Germany and Austria have been especially good at keeping the waters clear, he said.

Waste-water treatment projects and the phasing out of certain industrial chemicals had helped improve water quality.

But because such projects are slow and costly, they can take a long time to have an impact and must be accelerated in polluted areas to save plant and animal life, Mr Liskea added.

The second-longest European river after the Volga in Russia, the Danube flows through four national capitals - more than any other in the world - and is the source of drinking water for millions of Europeans.

The Danube, which became badly polluted in the mid-20th century, was also the inspiration for the famous waltz composed by Austrian Johann Strauss as he was travelling downriver - "An der schoenen, blauen Donau" (On the Beautiful Blue Danube).