Drone footage has shown hundreds of tonnes of "revolting" illegal waste, fly-tipped in a field next to the River Cherwell near Kidlington in Oxfordshire.
The Sky News video shows the mass of rubbish, which is at least 60m long, 15m wide and stacked 10m high, and weighs hundreds of tonnes.
Calum Miller, Liberal Democrat MP for Bicester and Woodstock, told Sky News it was the first time he had seen anything on this scale, questioning whether the Environmental Agency had the resources to deal with it.
The cost of removing the waste is estimated to be more than the entire annual budget of the local council, which is around £25m.
With the site on a floodplain, Mr Miller listed what he saw as the three major environmental risks - waste being washed into the waterways, rain seeping through the waste and carrying toxins into the water and the danger of decomposing chemicals presenting a fire risk.
He said the police had used a helicopter with a heat-seeking camera, and could see that some of the waste was indeed starting to decompose.
The site is adjacent to the A34, a busy road running through cities including Oxford and Birmingham.
Mr Miller said he believed the Environment Agency was first made aware of the issue back in July.
He said he believed it was the work of "organised criminal gangs" and raised a "bigger systemic problem around the country", with "dumps are cropping up in more and more places".
He went on: "My concern is the Environmental Agency lacks the resources to deal with criminal activity on this scale. I'm calling on the government to take action and ensure those who are dealing with such incidents have the powers they need to tackle it at source."
Friends of the Thames's chief executive officer, Laura Reineke, told Sky News she became aware of the dumping site around 10 days ago, calling it "the biggest ecological disaster that's happened on an inland waterway in this country".
The head of the charity dedicated to protecting, restoring and celebrating the River Thames, said the fact that the area had been prepared with large earth-moving vehicles and that the rubbish was "pre-shredded", then dumped so neatly showed the operation had been "very well organised".
The culprits are yet to be caught.
Ms Reineke went on: "What is most horrifying is that the Environment Agency have known about this since 10 September [at the latest], and they have done nothing about it."
She also questioned the Environmental Agency's management of the situation, calling it "a story of total incompetence".
Ms Reineke called the delay "a death sentence for the River Cherwell and the wider Thames catchment", labelling it "ecocide on an epic scale".
She added: "This dump isn't only a catastrophe for the freshwater species in our waterways, it's also a public health issue. We don't know what these chemicals are, they should have been tested."
The Environment Agency told Sky News it 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.
"We share the public's anger about incidents like this, which is why we take action against those responsible for waste crime," asking anyone with information to call their 24-hour incident hotline.
This summer, Sky News produced an exclusive report on the growing problem of waste crime, followed up in October by a report into an illegal dump in Wigan.
In a report released last month, the Lords Environment and Climate Change Committee warned that organised crime gangs are 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.
Philip Duffy, the agency's chief executive, hit back, saying: "I think it's very unfair on my hardworking staff to be accused of incompetence."