Hungary’s toxic sludge spill has reached the Danube River but may not harm wildlife after emergency services battled to neutralise the alkaline substance.

A million cubic metres of toxic sludge spilled from a reservoir at an alumina plant in Ajka, western Hungary, this week and has poured through several communities. The flood, which is larger in volume than the Deepwater Horizon gulf oil spill, left four people dead and injured more than 100.

Rescue crews attempted to stop the flow of the sludge before it reached the Danube but were unable to and the substance reached a tributary of the river on Thursday.

The Hungarian government says the incident is the worst “ecological disaster” in Hungary’s history, after the pH level of the sludge killed all wildlife in the Marcal, the first river it reached.

However, officials say that the alkalinity of the toxic sludge is declining after emergency workers poured huge quantities of clay and acid into streams to neutralise it, meaning that wildlife in the Danube should not be affected.

Tibor Dobson, a spokesman for the country’s emergency services, said: “The current pH level in the Danube is absolutely not dangerous for the flora and fauna of the river. It appears that we managed to save the river’s wildlife.”

However, countries downstream from Hungary, including Croatia, Serbia and Romania, are drawing up emergency plans to cope with the sludge if it reaches them via the Danube.

There are also concerns that a change in weather could have catastrophic effects. Recent rain has kept the sludge wet and officials now fear that warmer weather will create dust that could spread toxins, and possibly low-level radioactive materials.

If the sludge becomes toxic dust that can be spread through the air then people may need to be evacuated from the area.