Government must still implement the Nitrates Directive despite its "flaws", an influential committee of MPs has said.
A report published by the Environment, Food and Rural Affairs Select Committee (Efracom) said the legislation, which dates back to 1991, is old fashioned but is unlikely to be replaced in the near future so Defra must comply with it.
The UK is facing a legal challenge from the European Commission over its failure to fully implement the directive.
The move led Defra to propose a number of changes to the regulations in England last year which would change the way farmers store and spread livestock manure, and use chemical fertilisers.
If introduced, the changes could affect nearly 200,000 farmers.
Efracom said ministers must convince the Commission that the UK is complying with the directive to ensure the legal proceedings are brought to an end.
"The directive is imperfect but other member states have managed to implement it successfully and the UK will have to do the same," said committee chairman Michael Jack.
"Defra needs to strike a balance between satisfying the Commission that it is complying with the directive and convincing farmers that the changes are fair and proportionate. Then Defra could make the case for changes to the directive."
MPs said some of Governments's proposed changes are welcome and sensible but others need more work.
The requirement for increased storage space for slurry could impose heavy costs on dairy and livestock farmers, they warned, and said the Treasury should enhance tax allowances for the construction of facilities.
The National Farmers Union echoed the committee's concerns about some of Defra's proposals and welcomed its recommendation for tax allowances.
President Peter Kendall said: "We acknowledge Defra must act and we join the committee in calling for fair and well reasoned measures."
Commenting on the proposed changes to the Nitrates Directive, a Defra spokeswoman told edie: "Defra has undertaken an extensive consultation which prompted over 600 responses.
"Many of these were very detailed, and it is right that we consider them carefully.
"We recognise people's concern to know what the conclusions will be, and we will announce them as soon as possible. The Committee's report is timely because we can take it into account."
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