The River Wye will be in irreversibly worse condition within two years unless swift action is taken, politicians and regulators have been warned.

This was Wye and Usk Foundation chief executive Simon Evans’ message to a meeting last month of the Wye Nutrient Management Board (NMB), which brings together politicians, regulators, farmers and businesses to address the river’s worsening pollution problem.

“We are on the edge of a precipice,” he said afterwards. “We’ll see the Wye going green each year, people won’t be able to swim in it, dogs will be poisoned.”

He explained that the ecology of the Wye depends on an aquatic plant, water-crowfoot (Ranunculus) that is home to a huge number of “bugs and beasties”. These previously consumed algae on the river, which lately have caused lethal “blooms”.

Much of the Ranunculus was lost in the record floods of two winters ago, and the algae have had the upper hand since, leaving the main Wye now with less than 10 per cent of the plant compared to three years ago.

However the board began the process of having the entire Wye catchment designated a Water Protection Zone (WPZ), and this was backed by a motion agreed unanimously by Herefordshire councillors last Friday, which Evans said was “a huge step in the right direction”.

Under this, activities thought to be causing pollution can be restricted or banned altogether, backed by criminal penalties. But the designation can only be made by the environment secretary on the basis of evidence that existing measures have been ineffective.

One such zone is already applied to parts of the river Dee in Cheshire, and a second is being looked at for Poole harbour in Dorset.

Ross-on-Wye councillor Louis Stark urged his fellow councillors to get behind the move. “We inflict an injustice daily on our great river system, which has become a soup of pollutants,” he said.

“We need a whole-river system solution, covering all 155 miles of the Wye and its tributaries, to enable it to recover. Everyone has to do their bit.”

The council must now press agencies on both sides of the border to make the case to government for a WPZ, Coun Stark said.

“It raises the stakes. WPZ’s are already in law – we don’t need to reinvent the wheel. Funding now needs to come from Westminster or Cardiff, if they are serious about this.”

Seconding the motion, Coun Toni Fagan said: “When I first made Herefordshire my home, my children would swim in the Wye – it appeared we had dropped into a paradise. Twenty years later, the river has become a sewer.”

Coun Elissa Swinglehurst, who chairs the NMB, told the meeting that while she sympathised with individual farmers trying to address the problem, “voluntary measures are not being effective in reducing the overall contribution from agriculture”, and so “the time is right” for this more forceful measure.

Coun Felicity Norman said: “We see appalling examples of farming practice which adds to problem – piles of manure in wrong places, ploughing downhill, bare soil in winter that sloshes into rivers.

“The good farmers have to put pressure on the poor ones.”

Coun Terry James said that what has happened to the Wye “is the saddest thing to have happened to this county in my lifetime”, adding: “We need to say to government, ‘for God’s sake do something about it before it’s too late’.”

But Mr Evans said he thought there is now a “political tailwind” behind the issue, which has been raised in Parliament including at last week’s Prime Minister’s Questions.

Hereford and South Herefordshire MP Jesse Norman asked Boris Johnson to press agencies to properly tackle pollution in the river, to which Mr Johnson replied: “We’re urging the Welsh Government to take this matter as seriously as this Government is.”