The High Court ruled Friday that the UK government had acted unlawfully when it approved elements of its plans to meet net-zero targets based on "vague and unquantified" information.
It is the second time in two years that the court has ruled that ministers are not following the UK's own climate change laws. A judge handed down a similar ruling in 2022.
Prime Minister Rishi Sunak, in power since October that year, has faced persistent criticism that he is watering down the country's commitment to net zero targets after a series of climate policy reversals.
Friday's ruling was prompted by on a legal challenge by campaign groups Friends of the Earth, ClientEarth and the Good Law Project. It means ministers must redraft part of their net-zero plans.
"This is another embarrassing defeat for the government and its reckless and inadequate climate plans," Friends of the Earth lawyer Katie de Kauwe said.
The groups took joint legal action against the Department for Energy Security and Net Zero over its decision in March 2023 to approve the Carbon Budget Delivery Plan (CBDP).
The plan outlines how the UK will achieve targets in the country's so-called carbon budget, which runs until 2037, as part of wider efforts to reach net zero by 2050.
They argued that the relevant secretary of state at the time, Grant Shapps, lacked key information on whether individual policies could be delivered but approved them anyway.
That breached the 2008 Climate Change Act, which requires such due diligence on emissions-cutting targets and strategies, said the groups.
High Court judge Clive Sheldon agreed, calling the plan presented to Shapps for sign-off "vague and unquantified" and saying it lacked "sufficient" information.
"It is not possible to ascertain... which of the proposals and policies would not be delivered at all, or in full," he concluded.
A Department for Energy Security and Net Zero spokesperson insisted "the claims in this case were largely about process and the judgment contains no criticism of the detailed plans we have in place.
"We do not believe a court case about process represents the best way of driving progress towards our shared goal of reaching net zero," the spokesperson added.