The successful legal challenges were brought by Friends of the Earth, ClientEarth, Good Law Project and environmental campaigner Jo Wheatley and heard together at the Royal Courts of Justice in London in June.

In a judgment published last night amid the Met Office’s first ever red alert for extreme heat, Mr Justice Holgate found that the strategy falls short of 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.

The ruling states that Greg Hands, the minister for business, energy and industrial strategy, who was responsible for signing off the net zero strategy, approved the strategy without having the legally required information on how carbon budgets would be met.

The government must now update the strategy to include a quantified account of how its policies will achieve climate targets, based on a realistic assessment of what it actually expects them to deliver.

The updated strategy will have to be presented to parliament for scrutiny by MPs, and the judgment says the advice of Westminster’s climate change committee must be given “considerable weight”.

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.

“More than a decade ago, Friends of the Earth spearheaded the grassroots campaign that led to this vital piece of legislation. Today, we have strengthened its enforcement, which is so crucial to our country’s ability to tackle the climate crisis.”

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.”

Jolyon Maugham QC, director of Good Law Project, said: “The illegality of its flagship climate change strategy is a huge political embarrassment to the government. The net zero target must be a road map to a sustainable future — not a lie we tell our children.

“We are thrilled to have worked alongside our friends at ClientEarth and Friends of the Earth to deliver this landmark victory.”