Boris Johnson has promised “urgent action” on the climate crisis, taking personal leadership of this year’s UN climate talks after a blistering attack by the sacked former minister who was to lead them.

“Unless we take urgent action, we will get 3C hotter,” the prime minister told a gathering of climate experts, business leaders and civil society groups at the Science Museum in London on Tuesday morning. “As a country, as a society, as a planet and as a species, we must now act.”

He called on all governments to follow the lead of the UK in setting a target of net zero emissions by 2050, promised support for “our Chinese friends” in their efforts to tackle species loss and environmental degradation, and announced he would bring forward the phaseout of diesel and petrol cars in the UK from 2040 to 2035.

In a boost to Johnson as social media buzzed with criticism of his handling of the climate talks so far, Sir David Attenborough signalled his strong public support at the launch.

Play Video 1:49 'A huge encouragement': Sir David Attenborough endorses government's climate promises – video

“Unless we do something, [the climate crisis] becomes insoluble. That’s why Glasgow is extremely important,” said Attenborough. “It is up to us to organise the nations of the world to do something about it. That it is why it is so encouraging to know that the present government has devoted this year to it. It’s a huge encouragement to those of us who have been worrying about this problem for a very long time to know that now this government is going to do something about it.”

The UK will host this year’s crunch UN talks on the climate crisis, known as COP 26, scheduled for Glasgow this November. But the launch of the government’s strategy got off to a troubled start, as the intended president of the talks, the former energy minister Claire O’Neill, was abruptly sacked late on Friday.

Q&A What is Cop – and how will it help? Show Hide For almost three decades, world governments have met every year to forge a global response to the climate emergency. Under the 1992 United Nations Framework Convention on Climate Change, every country on earth is treaty-bound to “avoid dangerous climate change”, and find ways to reduce greenhouse gas emissions globally in an equitable way. Cop stands for conference of the parties under the UNFCCC. The UK will host Cop26 this November in Glasgow. In the Paris agreement of 2015, all governments agreed for the first time to limit global heating to no more than 2C above pre-industrial levels, and set out non-binding national targets on greenhouse gases to achieve that. However, these targets are insufficient, and if allowed to stand would lead to an estimated 3C of heating, which scientists say would spell disaster. For that reason, the Cop26 talks in Glasgow are viewed as the last chance for global cooperation on the emergency, with countries expected to come with tough new targets on emissions. The negotiations will be led by environment ministers and civil servants, aided by UN officials. Nearly every country is expected to send a voting representative at the level of environment secretary or equivalent, and the big economies will have extensive delegations.

Each of the 196 nations on earth, bar a few failed states, is a signatory to the UNFCCC foundation treaty. The Cops, for all their flaws, are the only forum on the climate crisis in which the opinions and concerns of the poorest country carry equal weight to that of the biggest economies, such as the US and China. Agreement can only come by consensus, which gives Cop decisions global authority. Fiona Harvey Environment correspondent

That leaves a vacuum that will not be filled, perhaps for as long as a fortnight, as the appointment depends on a wider cabinet reshuffle. Former prime minister David Cameron was approached but declined the role. Possible other candidates are understood to include Michael Gove, a serving cabinet minister, and the former Tory party leaders William Hague and Michael Howard, both now in the Lords.

O’Neill made a damaging intervention with a swingeing attack on the prime minister on BBC Radio 4’s Today programme on Tuesday morning, just before the launch. She accused Johnson of not understanding the climate emergency, of a lack of commitment and of being untrustworthy.

Whitehall sources have been briefing that O’Neill had rows with her staff and gave contradictory opinions on the COP 26 talks when meeting other governments.

O’Neill rebuffed these accusations in a letter to Johnson, saying: “No 10 is rumoured to be behind the media briefings put out to support your decision, which variously contained awful, false and distorted defamatory allegations. To take two examples: ‘bullying allegations’ were referred to, when you are aware that there was a single historical complaint, which was fully investigated by the Cabinet Office and found to be entirely without merit. Equally, reports of ‘problems on international engagements’ stemmed from a single blogpost which I believe can be completely rebutted by the emails.”

High-ranking officials and people familiar with O’Neill’s work as COP 26 president in the few months since her appointment have said her performance has been mixed, with some criticising her for falling out with senior officials. Others have said she made a good impression on other governments at key meetings.

Time is now running out, several people familiar with the talks warned. “We are certainly far behind where we should be now” in getting key countries on board, said one longtime observer of the climate negotiations.

Saving the planet: UK role vital if COP 26 climate talks to succeed Read more

The mission for COP26 is to forge a new global consensus on the climate crisis. At Paris in 2015, all governments agreed for the first time to limit global heating to no more than 2C above pre-industrial levels, and set out non-binding national targets on greenhouse gases to achieve that.

However, those national commitments are insufficient to meet the Paris goal, and if allowed to stand they would lead to an estimated 3C of heating, which scientists say would spell disaster. The world’s leading scientists, the Intergovernmental Panel on Climate Change, have warned that the world must drastically change direction in the next decade to have a hope of meeting the Paris goals.

For that reason, this year’s talks are viewed as the last chance for global cooperation on the emergency. Countries are expected to come to Glasgow with stringent new targets on emissions.

Johnson’s launch of the UK’s COP 26 strategy was intended to draw a line under the O’Neill row and move forward to an all-out diplomatic strategy to forge agreement among 196 nations, some of which are reluctant to come up with new targets and some of which are avowed opponents of the Paris accord.

However, without a replacement for O’Neill, it looks hard to persuade people that the UK has really started on what is seen as one of the biggest international challenges this year. As one prominent attendee at the launch said: “We are getting no direction. That’s what’s missing here.”