Operators of the historic Monmouthshire and Brecon Canal have warned it could start to dry up within a week due to an "urgent" water shortage.
The 225-year-old waterway is facing a crisis with no rainfall expected imminently.
Local businesses and tourism could be severely impacted if the canal, which attracts millions of visitors annually, begins to run dry.
The 35-mile waterway weaves through the Bannau Brycheiniog National Park - formerly known as the Brecon Beacons - and the Blaenavon industrial landscape World Heritage Site.
Operators of the historic Monmouthshire and Brecon Canal have warned it could start to dry up within a week due to an 'urgent' water shortage
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It attracts an estimated 3 million visitors a year for boating, walking and cycling.
"Everything the canal supports is at threat - we risk losing the jewel in the crown of south Wales," Tracey Leake-Jones, who runs the Bridge 46 to Five Locks canal group, told the BBC.
The canal has historically relied on funnelling water from the River Usk and its tributaries, which accounted for 80-90 per cent of its water supply.
Recent changes in legislation, designed to protect rivers from climate change impacts, now require licenses that restrict water abstraction.
Last year, Glandŵr Cymru, the Canal and River Trust in Wales, lost an appeal against conditions stipulated by environmental regulator Natural Resources Wales.
Alasdair Kirkpatrick, owner of Beacon Park Boats, said he had already received calls from concerned customers who had booked trips.
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"I never saw this one coming," he said. "It has really shocked us all."
He noted that his customers visit local pubs and restaurants, with 25 per cent flying in from overseas, "using local taxis and train services and spending a fortune locally."
The River Usk is a designated special area of conservation protected by law due to its importance for rare wildlife.
This includes the Atlantic salmon, which is threatened with extinction from Wales.
"The canal took around 30 per cent of the total flow of the river in 2022," said Gail Davies-Walsh, chief executive of Afonydd Cymru, which represents river groups across Wales.
The 35-mile waterway weaves through the Bannau Brycheiniog National Park - formerly known as the Brecon Beacons - and the Blaenavon industrial landscape World Heritage Site
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She added that limits were needed to ensure species and habitats had sufficient water to survive.
Mark Evans, director of Glandŵr Cymru, said the trust accepted it could no longer abstract water from the Usk at previous levels.
Discussions with Welsh Water have explored whether it could support the canal during dry periods.
Welsh Water has proposed using spare capacity from the River Usk when supplying some customers from the River Tawe.
However, this would come at a cost that the charity may not be able to afford.
A Welsh Water spokesman said providing water free of charge "isn't an option for us" as the Usk supplies drinking water to 250,000 customers.
The Welsh government has declined to intervene despite calls from businesses and community groups.
A spokesperson said any arrangement between the canal and Welsh Water would be "a commercial contractual decision in which the Welsh government should play no role."
