More than half the chalk streams and a quarter of rivers in England are at risk of drying out as too much water is being pumped out of them, wildlife experts have warned.
More than 550 bodies of water in England and Wales are being "over-abstracted", affecting rivers and chalk streams such as the Itchen in Hampshire, and London's River Cray, figures from Freedom of Information requests by the WWF suggest.
If too much water continues to be taken out of rivers for public supplies and agriculture, reducing their flows, it will hit wildlife such as threatened water voles and kingfishers, the conservation charity said.
The warning comes as parts of the UK face the possibility of drought following low winter rainfall, which the WWF said could push wildlife to the brink.
April was "exceptionally" dry, with most of the UK receiving less than half the average rainfall for the month, and southern England has seen the driest July to April in records stretching back more than 100 years.
Increasing weather extremes due to climate change, poor river management and over-abstraction is putting a significant proportion of streams and rivers at risk, and the situation is likely to get worse unless urgent action is taken, the WWF warned.
Tanya Steele, chief executive of WWF, said: "If we have a dry summer our green and pleasant land could become as parched as some of the Mediterranean.
"This may sound attractive for sun-seekers, but in fact it would be disastrous for wildlife.
"It would mean hundreds of millions of pounds of damage, tens of thousands of fish dying, and serious declines of some of our most loved wetland species.
"But this can be avoided if we update the outdated way we manage the demand on our country's limited water supplies."
The charity is calling for the Government to ensure all EU water and environmental rules are transferred into UK law to safeguard nature, and for urgent action at the 555 river water bodies where abstraction is already causing damage.
Water companies must set out long-term wastewater plans to stop sewage polluting rivers, and environmental regulators should be given the power to ensure all abstraction is limited to sustainable levels.
The WWF also wants to see a national strategy to reduce water waste.
In a campaign backed by rivers trusts and angling clubs, the charity is urging people to find out the state of their local river and ask their MP to put pressure on new Environment Secretary Michael Gove to protect the waterways.
Polling for the WWF reveals widespread support for Government to restrict the amount of water taken from rivers (69%) and for water companies to only abstract sustainable amounts of water (81%), while 90% thought the firms should do more to fix leaky pipes.
Four-fifths believe wildlife has as much of a right to water as people, and nearly seven in 10 (69%) agreed they were worried about the environmental impact of taking water out of rivers, the survey of 2,003 people by Populus found.
An Environment Department (Defra) spokeswoman said: "We recognise the need to improve water management.
"That is why we are committed to reforming abstraction licensing into a simplified, modern system that is fairer and more resilient.
"The Environment Agency has already made significant changes to over 250 abstraction licences and we are examining a range of ways to further protect the environment, including legislative or regulatory changes."
An Environment Agency spokeswoman said: "We are in a prolonged period of dry weather which began in the autumn last year and despite rain in May some rivers, groundwaters and reservoirs are still lower than normal for the time of year.
"The Environment Agency is working actively with water companies, businesses and farmers to balance the needs of water users and minimise any potential impacts to people, the environment and wildlife.
"Since 2008 we have made changes to over 250 abstraction licences to prevent over 27 billion litres of water per year being removed from the environment.
"Our latest assessment (2016) shows that 81% of rivers, lakes and estuaries have water flows that supports the ecology well."