Fly-tippers have dumped a “mountain of illegal waste” in Oxfordshire so large that removing it could cost more than the local council’s annual budget, the area’s MP has said.

Hundreds of tonnes of waste, stacked 10 metres high, appeared in a field between the River Cherwell and the A34 near Kidlington. One charity called the huge dump of rubbish “an environmental catastrophe unfolding in plain sight”.

Calum Miller, the Liberal Democrat MP for Bicester and Woodstock, told parliament the “estimated cost of removal is greater than the entire annual budget of the local district council”.

He added: “Criminals have dumped a mountain of illegal waste weighing hundreds of tonnes in my constituency on a floodplain adjacent to the River Cherwell. The Environment Agency said it has limited resources for enforcement.”

Friends of the Thames said the illegal rubbish dump was created about a month ago by an organised crime group. It said no visible containment or mitigation measures appeared to be in place.

The charity’s chief executive, Laura Reineke, said: “This is an environmental catastrophe unfolding in plain sight.

“A mountain of waste has been allowed to build up just metres from the Rver Cherwell – an illegal landfill sitting on the floodplain of one of our most important waterways.

“Every day that passes increases the risk of toxic runoff entering the river system, poisoning wildlife and threatening the health of the entire catchment.

“The Environment Agency must act now, not in months or years – which is their usual reaction time.”

Billy Burnell, a local angler who regularly fishes in the area, told the BBC he noticed the pile in September. He said it was “horrific” and called the potential runoff into the river an “environmental disaster waiting to happen”.

Anya Gleizer, a geography researcher at the University of Oxford, said: “What we have on our hands, right now, is an environmental and health emergency that threatens not only the Cherwell River and its ecosystem, but also poses a direct risk to us: the communities living downstream from the dump-site,” she said.

“As an ecologist, river guardian, rower but also, more simply, as a mother of a child that enjoys splashing about and fishing for aquatic invertebrates, I have to emphasise that this crime has more far-reaching consequences.”

The Environment Agency has obtained a court order to close the site to all public access for at least six months.

A spokesperson said: “Specialist officers are investigating waste dumped near the A34 at Kidlington. Their role will be to find who left the waste there and take appropriate action.

In a report released last month, the Lords environment and climate change committee said organised crime gangs were illegally dumping millions of tonnes of waste across the countryside every year.

The committee identified incompetence at the Environment Agency as a factor in the growing crisis.

But Philip Duffy, the agency’s chief executive, hit back, and said: “I think it’s very unfair on my hard-working staff to be accused of incompetence.”