A Tory minister named in a Privileges Committee report for interfering in Boris Johnson’s partygate probe has resigned from government.

Lord Zac Goldsmith, a former MP, has quit his environmental role, claiming Prime Minister Rishi Sunak was “simply uninterested” in the issue.

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On Thursday, the Tory peer – a close ally of Mr Johnson, who appointed him to the Lords – was chastised by the Privileges Committee for tweeting about its finding that the former prime minister had lied to parliament about lockdown breaking parties in Downing Street.

In the post, he characterised their inquiry as a “kangaroo court” and “witch hunt” – but Mr Sunak’s spokesman insisted the PM still had confidence his the minister.

However, 24 hours later, Lord Goldsmith resigned his post with a stinging resignation letter, taking aim at the current incumbent at Number 10.

The Tory peer said it had been an “exhilarating experience” in the job, praising the progress the UK had made in leading on climate change internationally – particularly when Mr Johnson was in Downing Street.

But Lord Goldsmith said he had been “horrified” by the government’s “abandonment” of policies around animal welfare, and that its efforts on environmental issues at home had “simply ground to a standstill”.

“More worrying, the UK has visibly stepped off the world stage and withdrawn our leadership on climate and nature,” he wrote in his resignation letter.

“Too often we are simply absent from key international fora. Only last week you [Mr Sunak] 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.”

Lord Goldsmith said the government had “effectively abandoned” its “solemn” commitment to spend £11.6bn of its aid budget on climate and environment causes – calling it “the single most important signal” to countries suffering and in persuading other G7 countries to act – and had “not come clean on the broken promise” as the final year of spending will be after the next general election.

‘Government apathy on climate’

“Prime minister, having been able to get so much done previously, I have struggled even to hold the line in recent months,” he wrote.

“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.

“I will never understand how, with all the knowledge we now have about our fundamental reliance on the natural world and the speed with which we are destroying it, anyone can be uninterested.

“But even if this existential challenge leaves you personally unmoved, there is a world of people who do care very much. And you will need their votes.”

The peer said It had been “a privilege” to hold his post, “but this government’s apathy in the face of the greatest challenge we have faced makes continuing in my current role untenable”, so he was resigning “with great reluctance… in order to focus my energy where it can be more useful”.

Former culture secretary Nadine Dorris – who was also named in yesterday’s report for her “vociferous attacks” on the Privileges Committee – responded to the resignation on Twitter, saying Lord Goldsmith’s “record of achievement”, “depth of knowledge” and “passion” were “second to none”.

She added: “We’ve just lost the most able minister for the environment any government would be lucky and proud to have. This loss is beyond party politics. It’s huge.”