An agricultural company responsible for polluting a Lincolnshire river, killing at least 100,000 fish, has been ordered to make plans to restore it.
Ammonia was released into the River Witham in March, causing the worst river pollution recorded in the county.
The Environment Agency (EA) said Omex Agriculture Limited was responsible and must now repair the damage.
An Omex spokesman said the leak was accidental and it was committed to making sure it would not happen again.
The ammonia affected the river from Bardney to the Wash, impacting the river and its ecosystems.
The EA has issued Omex Agriculture Limited with a notice of liability under the Environmental Damage (Prevention and Remediation) Regulations 2015.
It is only the second time such a notice has been issued since the regulations came into place.
Omex, which is based in Bardney, will be required to develop plans for repairing the environmental damage caused by the ammonia.
Manfai Tang, environment manager at the EA, said: "We know the pollution had a devastating effect on delicate river ecosystems, and issuing this notice means the process of helping the river recover to its natural, healthy state can start sooner.
"It's one way we're working to protect and improve our environment for local people and wildlife."
At the time of the leak, eyewitnesses described seeing some fish "gasping for breath".
Afterwards, EA experts and biologists worked to clean up the pollution and mitigate the damage.
The Omex spokesman said: "As soon as the incident was discovered the leak was stopped and repaired, and Omex has since been working with the Environment Agency to restore the river, and has provided funding for consultant ecologists to manage the repopulation of fish as quickly and sustainably as possible."
Fisheries teams have since restocked the river with more than 1.5 million fish larvae, including common bream, roach and tench. They are also re-introducing about 40,000 roach and common bream.