There were varying accounts of the amount of oil released: officials in Milan said they now believed 2.5 million litres had poured out, but environmental groups put the quantity at 600,000 litres.

The WWF wildlife charity said several water and bird species were at risk from the spill, with fish, wild ducks and herons – that were beginning to nest along the Po – the most at risk. Several oil-covered ducks have already been plucked from the river and taken for treatment at a regional animal shelter.

But even after the spill is cleaned up the impact will last, the WWF said, as the Po river valley is the most important agricultural region in Italy, and the Po is used extensively for irrigation, environmentalists noted. The spill began on Tuesday at the depot near Monza and spread south down the Lambro to Piacenza and Cremona overnight, despite efforts to contain it. By yesterday, it had reached the Po, which crosses the country from Piedmont in the west, across Turin and Ferrara, before emptying into the Adriatic.

"The scale of this is dramatic," said Damiano di Simine, regional president of the Legambiente environmental group. He said his organisation – as well as the regional government – had asked that a state of emergency be declared to release federal funds to help contain it.

The Lombard regional president, Roberto Formigoni, said those responsible would be prosecuted and punished severely for the "ecological disaster". "Some criminal decided to intervene in a harmful and cowardly way, putting at risk an asset that belongs to all of us," he was quoted as saying by the Apcom news agency.