Zac Goldsmith has resigned as a minister, accusing Rishi Sunak of being “uninterested” in the environment – but No 10 claimed he quit after being told to apologise for undermining the parliamentary inquiry into Boris Johnson. The Conservative peer and former MP published a long resignation letter detailing his disappointment with the prime minister for causing “paralysis” on the environment within Whitehall and choosing to attend the party of a media baron rather than an international environmental forum. However, Sunak hit back with a letter saying Goldsmith had been asked to say sorry for a tweet that appeared to challenge the privileges committee inquiry into whether Johnson misled parliament. The prime minister said Goldsmith’s comments were “incompatible” with his job in government and he had been asked to apologise but “decided a different course”. In response, Goldsmith said he had been happy to acknowledge he should not have commented on the privileges committee and denied this was the cause of his resignation. He said: “Our parliamentary democracy can only be strengthened by robust exchange and self-criticism and parliamentarians should absolutely be free to be critical of its reports and proceedings. “But as a minister it is true to say that I shouldn’t have commented publicly. No 10 asked me to acknowledge that and I was and am happy to do so. “My decision to step down has been a long time coming. When I compare what I and my amazing team were able to do before the current PM took office with the slow progress today, it seems to me I can no longer justify being in government.” Goldsmith’s accusations are likely to be unhelpful to Sunak in the lead-up to an election, with opposition parties expected to emphasise the government’s failure to do enough on the climate emergency, animal welfare and sewage discharges. Ed Miliband, the shadow energy secretary, called Goldsmith’s departure a “devastating indictment of Rishi Sunak and his whole government on climate and nature”. On Thursday the privileges committee named Goldsmith as one of the politicians who had undermined its work on whether Johnson misled parliament. The Liberal Democrats called on Sunak to sack Goldsmith because of the committee’s findings that eight politicians had put “improper pressure” on its inquiry. The committee highlighted that Goldsmith had retweeted a tweet calling the inquiry a witch-hunt and kangaroo court and added: “Exactly this. There was only ever going to be one outcome and the evidence was totally irrelevant to it.” In his letter stepping down from the Foreign Office, where his portfolio was overseas territories, Commonwealth, environment, energy and climate, Goldsmith said it had been a privilege to work as an environment minister, particularly under Johnson. He said he had been horrified by the Sunak government’s “abandonment” of policies around animal welfare, and that its efforts on environmental issues at home had “simply ground to a standstill”. Addressing Sunak directly, the Tory peer said: “Prime minister, having been able to get so much done previously, I have struggled even to hold the line in recent months. The problem is not that the government is hostile to the environment, it is that you, our prime minister, are simply uninterested. That signal, or lack of it, has trickled down through Whitehall and caused a kind of paralysis.” He added: “This government’s apathy in the face of the greatest challenge we have faced makes continuing in my current role untenable.” Goldsmith claimed the UK had “visibly stepped off the world stage and withdrawn our leadership on climate and nature”. He wrote: “Too often we are simply absent from key international fora. Only last week you seemingly chose to attend the party of a media baron rather than attend a critically important environment summit in Paris that ordinarily the UK would have co-led.” It is understood Goldsmith had been feeling uneasy about Sunak’s commitment to the environment since the beginning of his premiership, when the prime minister had to be cajoled into attending Cop27. The Conservative peer told friends at the time that he was considering resigning from a government that had seemingly deprioritised the environment and the natural world, a view that was cemented by a tussle over nature-based payments for farmers. However, he planned to stay in post for as long as it took to get the international forest agreement, agreed at Cop26, over the line. The deal between more than 100 leaders to save the world’s forests, partly made possible with funding from the UK, was seen as one of the great successes of the climate summit. This month the peer made up his mind to resign when Sunak’s government scrapped the kept animals bill after pressure from the hunting lobby, which was concerned that hunting hounds would be affected by its measures. The legislation would have cracked down on puppy farming as well as banning keeping primates as pets and banning live exports of farm animals. Friends say Goldsmith was “sickened” by the decision and wrote to Sunak begging him to reconsider, but that letter was ignored. After learning that international development funding to protect forests would not be honoured by Sunak, allies of Goldsmith say he made the final decision to quit. Goldsmith, a longtime ally of Johnson and his wife, Carrie, has resigned from a post before for environmental reasons, when he triggered a byelection over the issue of Heathrow expansion in his then seat of Richmond.