Water pollution goals are to be weakened by the government next week, the Guardian can reveal, as Environment Act targets will give farmers three extra years to reduce their waste dumping into waterways.

River campaigners have said the news is proof the government has not dropped its “attack on nature”.

Thérèse Coffey has been scrambling to release the legally binding targets mandated by the 2021 Environment Act, which gave the Department for Environment, Food and Rural Affairs (Defra) until October 2022 to set ambitious goals on air and water pollution as well as biodiversity.

In the act, the government gave itself a legally binding deadline of 31 October 2022 to provide “ambitious” targets on protecting air, water and biodiversity. The environment secretary is preparing to announce the targets at the end of next week, but the ambition for river pollution is set to be weakened.

Despite demands from water campaigners, there will be no overall target for river health.

It was also originally proposed that the agriculture sector would have to reduce pollution into waterways by 40% by 2037. This goal, according to leaked plans seen by the Guardian, has been pushed back to 2040.

Tim Farron, the Liberal Democrats’ environment spokesperson, said: “This is a scandal. Children are getting ill swimming in their local rivers whilst otters and fish live in filth, all because the government keeps kicking the can down the road. Conservative MPs refused to ban sewage discharges last year when there was a vote in parliament but surely now they have to grow some backbone by rebelling against this.

“Any watering down of these targets would be a betrayal to the public and environment. Ministers just don’t get it. In my mind, not setting targets for river health during a sewage crisis could be a resignation offence. I hope the environment secretary thinks twice about this”

James Wallace, the chief executive of campaign group River Action, said: “Agriculture is the biggest polluter of our rivers. We had hoped the new secretary of state, Thérèse Coffey, would end the attack on nature unleashed by her predecessor. Instead, her first move is to extend the deadline for unabated agricultural pollution to 18 years, further demonstrating this government doesn’t care about the water and nature emergency. Surely the politically salient strategy would be to boost investment in the environmental regulators and toughen industry targets for cleaning up our rivers.”

These targets have caused a headache for the new secretary of state. Dame Glenys Stacey, the chair of the Office for Environmental Protection (OEP), previously told Coffey that the possibility of taking formal enforcement action against the government over multiple missed targets was being kept under active review. The OEP can launch an investigation and take legal action if it deems it necessary.

Environmental charities including the RSPB, the Wildlife Trusts and the National Trust have also filed a complaint to the OEP and Defra over the failure to come up with new legally binding targets for air quality, water health, nature and waste management by the deadline.

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The government has caused widespread anger with its failure to tackle sewage and agricultural pollution, after cutbacks to farm inspection and an approach to polluting water companies described by critics as soft. Ministers were also forced into a U-turn on sewage pollution after initially whipping MPs to vote against a law to stop water companies dumping sewage. They then brought forward their own amendment that promised action on the sewage scandal.

The shadow environment secretary, Jim McMahon, said: “Only two months into the job, Thérèse Coffey has already broken her own government’s legal deadline to publish environment targets, told parliament that meeting sewage polluters isn’t a priority and now it appears she’s watering down and delaying action on tackling water pollution.

“Coffey’s first spell as a Defra minister was a monumental sewage spillage. It’s clear that Dr Dolittle is back to finish the job of polluting our environment. Labour has a plan to clean up the Tory sewage scandal. We will introduce mandatory monitoring with automatic fines, hold water bosses personally accountable for sewage pollution and give regulators the power to properly enforce the rules.”

A Defra spokesperson said nothing would be confirmed until the targets were published.