David Cameron has turned down an offer from Boris Johnson to head the UK’s preparations for a crucial international climate summit in Glasgow, saying he already has “a lot of things” to do this year.

The former prime minister was asked by Johnson to be the president of the 26th United Nations Climate Change Conference (COP 26) but Cameron rejected the offer.

Cameron said: “[There are] a lot of things I have already agreed to do this year, not least the work I do for Alzheimer’s Research UK, so I thought it was important that I carried on with that work.”

He called the climate change conference “absolutely vital”, adding that he wished the government well.

“I’m sure that there will be a government minister, or someone, who will be able to do the job and do it very well. The government has my backing as they go forward.”

Asked about his relationship with Johnson, Cameron declined to answer.

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, told BBC Two’s Newsnight on Tuesday that he understood Cameron felt “it was just a little too soon for him personally to come back into a frontline political role”.

The former Conservative leader William Hague was also reported to have been offered it and to have also declined.

On Tuesday during the conference’s launch event, Johnson refused to answer questions about who would take on the job.

The former clean growth minister Claire O’Neill, who stood down as a Tory MP at the general election, was sacked on Friday as president of the talks by the prime minister’s special adviser, Dominic Cummings. The government said the post would be a ministerial role in future.

The UN climate talks, to be held in Glasgow 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 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.

On Tuesday Johnson set out his vision for forging a new global consensus on the climate crisis, promising “we will crack it”.

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.”