The legal challenge was brought by environmental campaigners Friends of the Earth, ClientEarth, Good Law Project and private individual Jo Wheatley.
In its judgment, the court found that the Net Zero Strategy, which sets out plans to decarbonise the economy, doesn't meet the government's obligations under the Climate Change Act to produce detailed climate policies that show how the UK's legally binding carbon budgets will be met.
It also found that parliament and the public were effectively kept in the dark about a shortfall in meeting a key target to cut emissions.
The ruling states that Greg Hands, the minister for business, energy and industrial strategy, who was responsible for signing off the Net Zero Strategy, didn't have the legally required information on how carbon budgets would be met even though he approved the strategy.
The judge also concluded that the net zero strategy "lacked any quantitative assessment of the contributions expected to be made by individual policies to reductions in (greenhouse gas) emissions" and the report did not reveal that the analysis put before Hands left a "shortfall" against the required reductions, or of how that shortfall was expected to be met.
Now that it's been found unlawful, the government will have to update its climate strategy to include a quantified account of how its policies will achieve climate targets, based on a realistic assessment of what it expects them to deliver. The updated strategy will have to be presented to parliament for scrutiny by MPs.
The Climate Change Committee (CCC) recently found that credible plans exist for just two-fifths of the government's required emissions reductions. The judgment strengthens the critical expert role of the committee by stating that their advice must be given "considerable weight".
UK emissions are now almost half (47 per cent) their 1990 levels, but still rose by 4 per cent in 2021 as the economy began to recover from the Covid-19 pandemic.
During the court proceedings, it emerged that behind-the-scenes calculations by civil servants to quantify the impact of emissions cuts from policies in the government's Net Zero Strategy did not add up to the reductions necessary to meet the sixth carbon budget – the volume of greenhouse gases the UK can emit during the period 2033-37.
Friends of the Earth lawyer Katie de Kauwe said: "We're proud to have worked on this historic case. Taking strong action to cut carbon emissions is a win-win. Not only is it essential to preventing climate breakdown, but we can also tackle the cost of living crisis with cheap, renewable energy.
"This landmark ruling is a huge victory for climate justice and government transparency. It shows that the Climate Change Act is a piece of legislation which has teeth and can, if necessary, be enforced through our court system if the government does not comply with its legal duties."
Sam Hunter Jones, senior lawyer at ClientEarth, said: "This decision is a breakthrough moment in the fight against climate delay and inaction. It forces the Government to put in place climate plans that will actually address the crisis.
"The court has emphasised that the risks to delivery of the UK's climate targets are 'all- important' – the government must now address those risks when it prepares a revised strategy that meets the requirements of the Climate Change Act.
"This is also an opportunity to move further and faster away from the expensive fossil fuels that are adding to the crippling cost-of-living crisis people are facing. The decision confirms that the government must show how its plans will deliver the carbon budget targets in full.
"Its approach must also be realistic and based on what it actually expects its plans to achieve and the government must set out the emissions reductions expected from its individual policies, so that the public and parliament can properly hold it to account. This is a huge win for climate justice and accountability."
A BEIS spokesperson said: "The Net Zero Strategy remains government policy and has not been quashed.
"The judge made no criticism about the substance of our plans which are well on track and, in fact, the claimants themselves described them as 'laudable' during the proceedings."
