Rishi Sunak is likely to face a series of legal challenges aimed at thwarting his plans to U-turn on net zero policies amid further international condemnation of the proposals. Though the prime minister sought to shrug off criticism on Thursday, the UK’s independent climate watchdog joined the voices of concern, saying it was disappointed with changes that would make it more difficult for Britain to meet its legal commitments. One leading European politician said Sunak was turning the UK into “a climate villain and destroying its international reputation as a climate leader”. Campaigners including Friends of the Earth and The Good Law Project are now assessing how they can stop a rollback that would allow new petrol and diesel cars, and gas boilers, to be sold for longer. The two groups were already involved in legal action over the government’s climate plans, charging that they were inadequate. The Good Law Project has written to the energy secretary, Claire Coutinho, to warn of a fresh court challenge. In a letter seen by the Guardian, lawyers acting for the group have asked for answers within the next seven days to questions over how the government will meet its carbon goals while watering down and delaying green policies. Friends of the Earth has also written to the government. Niall Toru, the group’s senior lawyer, said: “Sunak’s decision to weaken UK climate policies will make it harder to meet our climate targets. Our lawyers will carefully scrutinise any new set of plans. Friends of the Earth has successfully taken legal action against the government’s climate strategy in the past – and we are prepared to do so again if Mr Sunak’s sums don’t add up.” Clean air campaigners are also considering a challenge. Rosamund Adoo-Kissi-Debrah, the campaigner whose daughter Ella died of air pollution, warned: “We will most likely end up in court again. [This is] a public health crisis as well as a green issue.” The Climate Change Committee (CCC), the statutory adviser on net zero, will scrutinise the prime minister’s proposals to judge whether they are in line with the UK’s legally binding obligations on greenhouse gas emissions. Piers Forster, the chair of the committee, warned that the policy rollback was likely to put the UK further behind on its targets. “We need to go away and do the calculations, but [Sunak’s] announcement is likely to take the UK further away from being able to meet its legal commitments,” he said. “This, coupled with the recent unsuccessful offshore wind auction, gives us concern.” Chris Stark, the CCC’s chief executive, warned on Thursday that he was “finding it hard not to be disappointed” by the government’s moves. “Earlier is generally better,” he said of policies to switch to electric vehicles and heat pumps. Michael Bloss, a German MEP from the Greens group, told the Guardian: “Rishi Sunak is becoming the leader of the fossil backlash. He is making the UK a climate villain and destroying its international reputation as a climate leader. These policies are destructive for the planet, which is already boiling, and they will be negative for the UK’s economy.” Javi López, a Spanish MEP from the centre-left group, said it is a “suicidal decision”. “Today’s efforts are tomorrow’s competitiveness.” Tim Crosland, an ex-barrister and the director of campaign group Plan B, which is not involved in the legal challenge, said the chances that the government could be taken to court were high. “This is very judicial review-able. There has already been a review of net zero which the government lost and their plans were ruled unlawful. Now, instead of improving its plans, it has done the opposite, so from a legal perspective this can certainly be challenged.” Tom Burke, a veteran adviser to governments and co-founder of the environmental thinktank E3G, said legal challenges would have been foreseen by the prime minister and might be part of his plan. “They want to generate outrage, like protests from Extinction Rebellion,” he said. “They will use legal challenges as an excuse to say the judicial processes must be reformed.” Ministers, including the home secretary, Suella Braverman, have made vitriolic attacks on “lefty” and “activist” lawyers, whom they accuse of using legal processes to subvert government aims. In Burke’s view, they might welcome the opportunity to launch similar, politically divisive attacks over net zero, if taken to judicial review. Political insiders believe the purpose of Sunak’s change of tack is to drive a “green wedge” between the Conservatives and Labour. Isaac Levido, his strategy guru, has pushed the line that the prime minister is “saving hard-pressed families from unacceptable costs” of green policies, while Labour would pile them on. Undeterred by the torrent of criticism from business, civil society, scientists and international observers that followed his announcements on Wednesday, Sunak has vowed to press ahead with the rollbacks. He told the BBC’s Today programme on Thursday: “I’m very happy to have opinions and advice from everybody, and everyone’s entitled to their view. We’re very confident – being in government, with all the information at our disposal – that we are on track to hit all our targets.” Communications from the Conservative campaign headquarters on Thursday suggested the party was also seeking to use recommendations by the independent CCC as a line of political attack. In an email headed “Questions to Labour on net zero”, it suggested to journalists a list of CCC recommendations including road taxes, flight taxes and carbon taxes as “questions to answer” that should be posed to Labour. Sunak repeatedly insisted his plans would save money for households and that he was still committed to the UK’s 2050 net zero target. But experts warned that many aspects of the plans were likely to add to costs for consumers. For instance, private renters will pay £1bn a year more in energy bills because of the scrapping of proposals to make landlords upgrade insulation on their properties, according to the Social Market Foundation. Pushing back the deadline for phasing out sales of new petrol and diesel cars will also cost consumers money, as electric vehicles save people £5,000 to £8,000 in fuel costs over their lifetime, the thinktank said. Though many of the changes Sunak announced will not have to pass through parliament, one may face a vote. The delay of the ban on new petrol car sales to 2035 will be done through an affirmative statutory instrument. Some of the Tories who have voiced concern at Sunak’s plans, including members of the Conservative Environment Network of more than 100 MPs, may choose to vote against it. But with the party trailing behind Labour in the polls, most are less concerned about the policy itself and more about whether it will make any difference with the electorate. “It’s predominantly a tidying up exercise. I’m not sure this is the big idea that is going to win us the election,” said one. Others questioned whether it would be as effective a political dividing line with Labour ahead of the next election as party strategists hoped. “The explosive response to the leak shows that it’s going to be tough for us to handle if we decide to make an issue of it,” one MP said.