Boris Johnson has set out his vision for forging a new global consensus on the climate crisis promising “we will crack it”, amid news that he approached former prime minister David Cameron to lead the UK’s preparations for a crucial summit.

Johnson has brought forward the UK’s phaseout of diesel and petrol vehicles by five years to 2035, and hastened the phaseout of coal-fired power by a year to 2024. He reaffirmed the UK’s pledge to switch to a net-zero emissions economy by 2050, and urged other nations – without naming any – to do the same.

“I hope that we can as a planet and as a community of nations get to net zero within decades,” Johnson said at the COP 26 launch on Tuesday. “We’re going to do it by 2050, we’re setting the pace, I hope everybody will come with us. Let’s make this year the moment when we come together with the courage and the technological ambition to solve manmade climate change and to choose a cleaner and greener future for all our children and grandchildren.”

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But the troubled start to the UK’s presidency of this year’s crunch UN climate talks with the sacking of Claire O’Neill as COP president has unsettled observers, who are hoping Johnson will play a pivotal role in bringing world leaders together with a new resolve to drastically reduce greenhouse-gas emissions before it is too late.

On Tuesday night it emerged that Johnson had approached Cameron to take over from O’Neill but he declined the role.

Lord Barker of Battle, who served as an energy and climate change minister under Cameron and is a close friend and ally of the former prime minister, said he understood reports that he was offered the role to be correct. “My understanding is that he felt it was just a little too soon for him personally to come back into a frontline political role,” he told BBC Two’s Newsnight.

William Hague was also reported to have been offered it but also declined.

The appointment last year of O’Neill as president – the official who will take the leading role in convening and chairing the fortnight-long UN talks, and seeing them through to a deal which will require the consensus of 196 nations – appeared to give the UK a good headstart in the talks.

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O’Neill was formerly an energy minister in the Department of Business, Energy and Industrial Strategy, below the secretary of state level but with rights to attend Theresa May’s cabinet meetings. She resigned as the MP for Devizes, a staunchly Conservative seat, after taking on the role, saying she wanted to concentrate on the COP presidency.

Brexit was also a major factor. O’Neill campaigned to remain in the referendum, and was scathing of hard Brexiters, whom she accused of being “hysterical” and “like jihadis”. She rebelled to vote for a Brexit bill amendment that would give parliament the final say on any deal to leave the EU, though she voted with her party on other legislation.

She wished Johnson good luck with Brexit when she gave notice last September, well before a election had been called, that she would not contest the seat again.

O’Neill was a surprise choice for the president’s role, but the political turmoil last autumn made her a relatively safe pick: with the government preoccupied with Brexit, and Johnson both struggling to get the general election he wanted and filling the cabinet with pro-Brexit supporters of his leadership campaign, to appoint a serving minister would have been tricky.

Other possible candidates included Zac Goldsmith, whose shaky reelection prospects were borne out by his defeat in Richmond, or members of the House of Lords.

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O’Neill’s experience as a transport minister and energy minister seemed to offer some assurance. However, her record was also spotted. In November 2018 three unions wrote to the Department for Business, Energy and Industrial Strategy to raise allegations of shouting and swearing at civil servants. In her post-sacking letter this week, O’Neill rebuffed those allegations.

The Guardian was given mixed reports of her conduct in the UN negotiations by people present. She is said to have created a good impression among some countries, and at some meetings at last December’s climate conference in Madrid. However, she fell out with some senior officials in the UK, and gave conflicting messages about the UK’s position and strategy at the talks.

O’Neill’s letter to Johnson also gives clues to the concerns over her conduct. In it, she is strongly critical of the COP structure, rules and bureaucracy, but without showing much awareness of what supporters see as the value of the process – which gives all nations an equal voice, and progresses by consensus – or respect for the negotiators, many of whom have served their governments for decades.

“COP is difficult, but you need to understand it and work within it,” said one long-time participant in the UN talks. “The French did [when they led the 2015 Paris agreement].”

Another former high-level diplomat and COP veteran said: “A good COP president makes all the difference between success and failure. They direct the negotiations, they play the key role in determining the outcome.”

It is also clear, however, that O’Neill has suffered from a lack of support within government, and a lack of focus from the prime minister on the UK’s COP 26 plans. She complained that the cabinet sub-committee on climate, supposed to be chaired by Johnson, has not yet met. The relationship between the COP 26 unit and other government departments is also unclear in parts, and it is not apparent that the Foreign and Commonwealth Office has been pushing climate to the top of the agenda in its embassies around the world, as the French did before Paris.

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Bedevilling all this has been Brexit. Several NGOs and developing country representatives told the Guardian about serious concerns that the UK could not give COP 26 the attention needed while still working out its new relationships and trade deals.

Some observers see a potential conflict of interest, as British diplomats seek at once to gain support for a COP 26 resolution that will require other countries to set out stretching goals on cutting emissions, while also negotiating post-Brexit trade deals. “I am very concerned about how they can play this,” said the head of one international civil society group. “This is a very delicate dance.”

Paul Bledsoe, a former climate advisrr to Bill Clinton, said: “Sacking O’Neill and making the post more directly reportable to Number 10 increases the pressure on Johnson to appoint an aggressive climate policy figure, especially one who will actually hold the Chinese and Americans accountable.”

With O’Neill’ssacking, climate activists and COP participants are hoping that now Johnson and his government can move on to forge a clear strategy and timetable for gaining the support and buy-in they need from capitals across the world. But they warn that the prime minister is running out of time.

“The prime minister has given a very clear and strong message, which is good,” said Lord Stern, the climate economist. “He has made a personal commitment, and that is now crystal clear. Now we need someone in a very senior position to be COP president. The challenge now is to accelerate.”

• This article was amended on 6 February 2020 because an earlier version referred to O’Neill’s “resignation letter”. As the article and that letter makes clear, she was sacked from her post.