Public prosecutor Felice Casson had said the workers fell ill because of exposure to vinyl chloride monomer (VCM), a toxic chemical produced in the plants and which is normally used to make plastic.

Casson said the management of chemical firms based in Porto Marghera - which included Eni's chemical divisions EniChem and EniMont, Montedison and Montefibre - had not protected workers although it was aware of the danger.

But the court ruled that the 28 company officials were innocent because the danger to health of VCM was not discovered until years after the workers fell ill.

Environmental groups reacted to the ruling with fury and some of the dead workers' relatives yelled "assassins" after the verdict was read out in a packed north Italian court room.

"This is a licence to kill...This is total deregulation. We live in a Far West where human life does not count for anything," said Grazia Francescato, leader of the Green Party.

Senior Venice city council official Gianfranco Bettin, known for his environmental campaigns, burst into tears.

The prosecution had requested jail sentences ranging from four to 12 years for the company officials, and the Italian state had filed a $37.42 billion lawsuit for environmental damage.

But the court ruled that the dead workers had contracted the illness before 1973, when the dangers of VCM were first known. It said fewer than 10 of them had died of liver cancer, which doctors link to exposure to VCM.

"The trial has ascertained that all the illnesses caused by VCM are linked to strong exposures (to the substance) during the 1950s and 1960s, when the toxic contents of VCM were not known," said Court President Nelson Salvarani.

He said that in 1973 the companies had acted immediately to reduce workers' exposure to VCM to ensure it remained within the limits set by lawmakers at the time.

Salvarani said the Venice lagoon had been polluted in the years before the introduction of environmental protection laws. He said the pollution, though "relevant", was now below the level that was dangerous for nearby residents.