A water company has been asked to nearly halve the amount of water it takes from a Hampshire chalk stream.
The Environment Agency has published a plan to reduce abstraction from the River Test from 136 to 80 million litres a day.
It said the change was needed to protect wildlife and to "restore sustainable abstraction".
Southern Water said it recognised the region was "water-stressed" and would need time to find solutions.
The proposed licence change would also restrict abstraction when the river flows fall below 355 million litres a day, compared to 91 million litres currently.
The "hands off flow condition" would be further increased from the year 2027.
In June, the World Wildlife Fund warned in a report that a combination of drought and water abstraction could cause serious harm to chalk streams.
Hampshire and Isle of Wight Wildlife Trust said the River Test was a "globally rare" chalk stream, and designated a Site of Special Scientific Interest.
Southern Water is licensed to obtain nearly half of south Hampshire's public water supply from the Test, with the remainder coming from groundwater and the neighbouring River Itchen.
It said the Environment Agency had already asked to vary all five abstraction licences in Hampshire, at a time when house building and population growth was increasing in demand.
A public inquiry in 2018 is due to explore previously requested licence changes on the Test and the Itchen.
Ahead of the inquiry, Southern Water has shelved plans for a new 19km (12 mile) pipeline to pump up to 45 million litres a day from the Test to the Itchen at Otterbourne.
It said the pipeline was needed to supply customers in east Hampshire in times of "extreme drought".