Boris Johnson’s government could be challenged in the courts over breaching its own climate change laws with a £29bn road expansion programme.

It comes after a landmark judgment at the Court of Appeal ruling plans for a third runway at Heathrow were illegal as ministers had not adequately taken into account the legally binding commitments to combat the climate emergency.

No 10 said it would not be appealing the decision – dealing a potentially fatal blow to the scheme – that said the ministers had not properly considered its impact on the Paris climate agreement.

During December’s general election, the prime minister vowed to invest £28.8bn in strategic and local roads across the country, but the proposals could now face legal action in the wake of the ruling against the expansion of the west London airport.

The BBC said the plans, which are due to be published next month, failed to take into account the legally binding commitment to net-zero carbon emissions by 2050.

The report suggested the current assessment of the lavish investment in roads across the country was done under guidelines last updated in spring 2019 – several months before Theresa May’s administration introduced the legally binding target to eliminate carbon emissions as one of her last acts as prime minister.

Transport Action Network – a group opposing “damaging road schemes” – told The Independent it is now seeking legal advice “to see whether the government is also acting outside the law in pursuing its road-building programme”.

The group added: “This programme is not only putting our ability to meet net zero by 2050 at risk but it also threatening our unique historic and natural environment.

“The government now needs to re-evaluate both its policy and spending priorities and get us back on track to deliver net zero by 2050 or preferably sooner.”

David Timms, head of politics at Friends of the Earth, added: “Following the incredibly significant judgment yesterday the government should now signal an end not just to airport expansion but also damaging road schemes. What’s needed are strategies that take into account, and are consistent with, our international climate commitments under the Paris agreement and sustainable development.”

He added: “In the year that the UK hosts vital UN climate talks, and with communities across the UK suffering extreme flooding, it’s clear we need transformative action to deal with the climate crisis. It is now time that our international climate commitments took centre stage in all decisions – including the forthcoming Budget.”

The Green MP Caroline Lucas said if the government’s commitments on climate change “are to mean anything” then all airport expansion should be stopped.

“As should massive road-building projects,” she said. “Instead we must invest in the infrastructure which will protect us from the worst impacts of the climate emergency and cut our carbon emissions.”

Stephen Joseph, a visiting professor at the University of Hertfordshire, also told the BBC: “It’s highly likely that the government will face further legal challenges if it goes ahead with road spending without having properly considered the implications for net-zero climate emissions.”

A spokesperson for the Department for Transport said: “We take our commitment to carbon targets immensely seriously and have one of the world’s most ambitious plans for reducing carbon emissions. We have developed our roads strategy alongside this.

“We’re also working hard to reduce the environmental impact of cars on our roads, with our ambitious plans to encourage the uptake of battery electric vehicles resulting in one sold every 15 minutes last year.”