The former president of the UN climate summit in Glasgow, who was sacked by the Government last week, has launched a blistering attack on Boris Johnson's record on tackling climate change.
Former clean growth minister Claire Perry O'Neill spoke out as the PM prepared to outline new measures, including a ban on sales of new petrol and diesel vehicles being brought forward to 2035.
In a letter to Mr Johnson published by the Financial Times, Ms O'Neill told him: "You promised to 'lead from the front' and asked me what was needed: 'Money, people, just tell us!' Sadly these promises are not close to being met."
She added: "This isn't a pretty place to be and we owe the world a lot better."
She said the PM had not convened the Cabinet subcommittee on climate change that he had promised, adding that the Government was "miles off track" in setting a positive agenda for the COP26 summit in Glasgow in November, and that promises of action "are not close to being met".
Her attack came as Mr Johnson vowed to urgently tackle the climate crisis as he launched the crucial summit.
He refused to answer who would replace Ms O'Neill but said: "We know as a country, as a society, as a planet, as a species, we must now act. We must reverse the appalling loss of habitats and species, it's only by repairing the damage to the natural world and restoring the balance between humanity and nature, that is now so grotesquely out of kilter, we can address the problem of climate change."
Downing Street also tried to avoid being further drawn into the row.
The Prime Minister's official spokesman said: "I have no comment to make at all in relation to the ... letter beyond saying the PM is grateful to Claire for her work preparing for what will be a very successful, ambitious climate change summit in Glasgow in November."
Asked if Boris Johnson understood climate change, the Prime Minister's official spokesman said: "You can see his speech today.
"He has positioned the UK as a world leader on tackling climate change with the most ambitious environmental programme of any country on earth."
As part of the UK's moves to meet its legal goal to reach net zero by 2050, the Government will consult on bringing forward a planned ban on the sale of new petrol and diesel vehicles from 2040 to 2035 - and earlier if feasible.
The ban, which Government adviser the Committee on Climate Change has called for by as early as 2030, will also include hybrid vehicles for the first time.
At an event attended by Sir David Attenborough, Mr Johnson called for international efforts to reach net zero as early as possible through investment in cleaner technology and protection of natural habitat - which will also help reverse losses in wildlife.
The event will also kick off a year of climate action across the UK, the Government said.
The UN climate talks in November are the most important since the Paris Agreement to curb global warming was secured in 2015.
Countries are expected to deliver more ambitious domestic plans for cutting greenhouse gases by 2030 - as current proposals are not enough to prevent dangerous temperature rises.
Pressure is also on countries to set out long-term plans for cutting emissions, with the science now clear that the world must reduce greenhouse gases to zero in a matter of decades to avoid the worst impacts of climate change.
The run-up to the talks will require a major diplomatic effort from the UK to secure ambitious climate action from countries - at a time when Britain is also negotiating trade agreements with the EU and other nations.
Ms O'Neill, who stood down as a Tory MP at the general election, was sacked as president of the talks by the PM's special adviser, Dominic Cummings, on Friday, with the Government saying the post would be a ministerial role in future.
But Nick Mabey, chief executive of climate change think-tank E3G, said the UK's presidency had got off to a bumpy start with the decision to dismiss her.
Her replacement has a "daunting task" and will need to be a political heavy hitter, with an immense diplomatic effort needed from the UK to steer the negotiations, he said.
While the UK has a legal target to cut emissions to net zero by 2050, the Committee on Climate Change has warned that domestic action to slash carbon dioxide and other greenhouse gases is lagging far behind what is needed - even before the net-zero target was set.
AA president Edmund King said the new target on car sales is incredibly challenging.
He added: "We must question whether we will have a sufficient supply of a full cross-section of zero- emissions vehicles in less than 15 years."
He also raised concerns that hybrids would be excluded from sale under the plans.
In her letter to Mr Johnson, Ms O'Neill suggested his personal animosity towards Scotland's First Minister Nicola Sturgeon is endangering the success of the summit and claimed the Prime Minister is considering relocating the event to an "English location" because of "ballooning costs".
Ms O'Neill is also reportedly consulting lawyers about what she claims were "false, distorted and defamatory" briefings about her record, saying Number 10 was "rumoured" to be behind the briefings.
She told the Financial Times she had been considered for a peerage by Downing Street to give her more authority in her UN summit role, but this had now been "firmly rescinded".