Tests have shown the presence of toxic hydrocarbons in the River Odra, which are not of natural origin but a technological by-product, two opposition MPs said on Monday in Olsztyn, northern Poland.
With climate change meaning hotter summers and low levels of water in one of Poland's biggest rivers, experts say that it was only a matter of time before something happened.
A group of scientists has appealed to the government for a new river management policy to prevent a repeat of the environmental disaster that has affected the River Odra.
Hundreds of firefighters, policemen and soldiers deployed to the Odra have recovered over 100 tonnes of dead fish as scientists continue to scramble to find the cause of the catastrophe.
Mateusz Morawiecki, Poland's prime minister, said on Monday that the government will look for ways to tighten laws against the illegal dumping of waste into rivers while promising harsh punishments for those found guilty.
The results of Polish and German tests of water samples taken from the River Odra, where masses of dead fish had been discovered, are the same, Poland's deputy climate and environment minister has said.
Four miners rescued from last Wednesday's coal mining accident at the Pniówek coal mine, in Poland, were in critical condition, doctors from the burn treatment clinic where the men were hospitalised reported on Monday.
Poland's president, Andrzej Duda, has expressed his sympathy to the families of four miners and a member of a rescue team who lost their lives in a mining disaster early on Wednesday morning.
Poland's foreign minister Zbigniew Rau expressed his sympathies to the Czech Republic after at least three people were killed and several hundred injured by a tornado and severe storms that swept through villages in the country's Moravia region on Thursday.