Environmental groups claiming to represent 20 million people will mobilise their members if UK ministers water down climate commitments, they have warned. Groups including the RSPB, National Trust and the RSPCA have written to the prime minister, Rishi Sunak, who has signalled his willingness to back away from green policies should the Conservatives stand to benefit from it electorally. “We will not stand by whilst politicians use the environment as a political football. It is courage and leadership that we need now,” they said. “In the past, we have mobilised many of our members collectively with extraordinary results, and our resolve to stand firm now against any and all attacks on this critical policy agenda remains absolute.” Their warning comes after the Conservative party narrowly succeeded in holding on to the safe Uxbridge and South Ruislip seat in the byelection triggered by the former prime minister Boris Johnson’s departure from the Commons. The constituency’s Conservative candidate, Steve Tuckwell, centred his campaign on opposition to the planned expansion of London’s ultra-low emission zone (Ulez), which is designed to reduce vehicle pollution in the capital, but which has also become a rallying point for opponents of the Labour London mayor, Sadiq Khan. The result led some to suggest that there was political capital to be made for Tories concerned about the party’s disastrous polling in exploiting so-called local wedge issues. Despite polls showing broad public support for action on the environment, the perception that successfully mobilising anti-Ulez campaigners in Uxbridge to vote for the Tories – or, at least, to not vote for their opponents – has led some to identify the environment as one such wedge issue. Since the byelection, Sunak has signalled that the government’s approach to its 2050 net zero commitment will be malleable under his leadership, saying he will follow a “proportionate and pragmatic” course that “doesn’t unnecessarily give people more hassle and more costs in their lives”. In their letter to him, the coalition called for an urgent meeting to discuss the climate and environmental crises, as well as “public reassurances” on his intention to take action. “There is no public mandate for a delay. It is, therefore, with deep alarm that we have read reports over the last few weeks of your government considering watering down its commitments on almost every front of environmental policy,” they told Sunak. Speaking to BBC Radio 4’s Today programme on Saturday, the Woodland Trust’s chief executive, Darren Moorcroft, said it was “incumbent” on them to act. He said signatories to the letter would not be shy in speaking out in the run-up to the general election. “We will make our voices heard with regard to how people should view any political party as it runs into the general election on what it is doing for the environment. So, instead of backtracking on environmental policies, we believe every political party that’s serious about winning should be setting out plans to get those good green jobs, to get cleaner air and seas, to restore our natural environment.” The letter will also serve as a warning to the Labour leader, Keir Starmer, who reacted to his party’s failure to secure Uxbridge and South Ruislip by urging Khan to ”reflect” on the planned extension of Ulez.