The prime minister admitted he "doesn't really get" climate change, the former head of this year's key summit on the issue has said.
The UK is hosting COP26 in Glasgow in November - but Boris Johnson sacked president Claire O’Neill on Friday.
Mrs O'Neill told the BBC there was a "huge lack of leadership and engagement" from the government.
But senior cabinet minister Michael Gove said Mr Johnson was dedicated to environmental issues.
Mr Gove told BBC Radio 5 Live that the prime minister described his political outlook as that of a "green Tory" when they first met 30 years ago.
"Ever since then I've seen his dedication to ensuring that we fight to ensure that our Earth is handed on in a better state to the next generation," he said.
But Ms O'Neill, the former Conservative minister for energy and clean growth, said people should be wary of the prime minister's promises.
"My advice to anybody to whom Boris is making promises - whether it is voters, world leaders, ministers, employees, or indeed family members - is to get it in writing, get a lawyer to look at it and make sure the money's in the bank," she told BBC Radio 4's Today programme.
"The prime minister has made incredibly warm statements about this over the years. He's also admitted to me that he doesn't really understand it. He 'doesn't really get it', I think is what he said."
She said the UK's climate efforts were at "Oxford United levels when we need to be Liverpool if we are going to do what the world actually needs us to do".
In a letter to Mr Johnson after she was sacked, Mrs O'Neill accused him of promising money and people, but failing to deliver either.
Mrs O'Neill wrote: "The cabinet sub-committee on climate that you promised to chair, and which I was to attend, has not met once.
"In the absence of your promised leadership… departments have fought internal Whitehall battles over who is responsible and accountable for (the conference)".
She said at this stage, the UK should have clear actions to communicate to the diplomatic network, an agreed plan of ministerial international engagements led by the prime minister and a roadmap for the proposed "year of action".
"As of last Friday, we did not," she said.
Downing Street declined to respond to the claims, with a spokesman saying only that the prime minister is "grateful to Claire for her work preparing for what will be a very successful, ambitious climate change summit in Glasgow in November."
The timing of the letter could not be worse as Mr Johnson is launching his strategy for the conference on Tuesday with the help of Sir David Attenborough.
Mrs O’Neill's letter focused not on UK policy, but on the state of international negotiations and of Mr Johnson's role.
She warns: "We are almost out of time to win the battle against climate change and start the process of climate recovery."
"It became clear to me that the current format of the global talks needed to be re-energised and focused.
"The annual UN talks are dogged by endless rows over agendas, ongoing unresolved splits over who should pay and insufficient attention and funding for adaptation (to inevitable climate changes).
"It was particularly awful at the last conference in Madrid. While half a million climate action protesters gathered in the streets, I sat in plenary sessions where global negotiators debated whether our meeting should be classified as 'informal' or 'informal-informal'."
She added: "There is a yawning gap between what the world expects from us and where we are. It's a systemic failure of global vision and leadership."
Mrs O’Neill recommended:
- Setting net zero emissions as the target for all climate ambition from countries, businesses, states and cities
- Introducing a "properly-funded" global package for adapting to inevitable changes in the climate
- Placing nature-based solutions (such as forest conservation) at the heart of the agenda
- New net zero sector deals from hard-to-decarbonise sectors such as cement and chemicals
Her comments are not just aimed at the government.
'We owe the world'
She criticised some climate negotiators, too, for refusing to accept that the annual parade of climate conferences will not deliver the cuts needed for a stable climate.
She said: "For some it is hard to give up on incrementalism even when it is demonstrably failing.
"In my judgement, this isn’t a pretty place for us to be to be and we owe the world a lot better."
Her words are likely to resonate round the world, although she is not the first climate diplomat to express this sort of frustration - and she’s unlikely to be the last.
Follow Roger on Twitter @rharrabin