The former head of the UN's climate change conference, Claire Perry O'Neill, claimed Boris Johnson had admitted that he "doesn't really get" the issue as she warned anyone receiving promises from the PM should be "in writing," checked by a lawyer and the funds secured before taking it seriously.
The former energy minister had been due to head the so-called COP26 meeting of world leaders in Glasgow in November but was sacked by advisor Dominic Cummings on Friday due to her not being a minister anymore.
She sent a letter to the Prime Minister accusing Number 10 of briefing "awful, false and distorted defamatory allegations" to the media to justify the decision and warning that the UK is falling back in the fight against climate change.
Speaking to the BBC Radio 4 Today programme Ms Perry O'Neill said that Boris Johnson had admitted to her that he did not really understand the issue of climate change.
"The Prime Minister has made incredibly warm statements about this over the years," said.
"He has also admitted to me that he doesn't really understand it. He doesn't really get it, I think is what he said."
She added: "We have seen a huge lack of leadership and engagement.
"Our efforts right now are somewhere around the middle of League One. We are playing at Oxford United levels when we need to be Liverpool if we are going to do what the world actually needs us to do."
Of the Prime Minister, Ms O'Neill said: "My advice to anybody to whom Boris is making promises - whether it is voters, world leaders, ministers, employees or indeed to family members - is to get it in writing, get a lawyer to look at it and make sure the money is in the bank."
Senior Cabinet minister Michael Gove rejected the claims by Ms O'Neill arguing that the Government is taking steps towards countering the threat of climate change.
Mr Gove, a former environment secretary, told Sky News: "The Cabinet has discussed climate change. The very first item on the agenda in the new year when the Cabinet met was the conference in Glasgow in November.
"We have been working hard in order to ensure we do three things - that Britain sets an example. The Prime Minister is explaining today alongside David Attenborough some of steps we are taking like, for example, banning the sale of petrol and diesel cars from 2035.
"The second thing is making sure countries come together and live up to their promises to reduce carbon emissions and the third thing is extending the range of things that we do to deal with this so it is not just about transport and energy."
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