Boris Johnson should set out plans to provide Covid-19 vaccinations to all developing countries to achieve a global climate deal, Labour’s shadow business secretary, Ed Miliband, has urged.

Only 2% of the population of developing countries have been inoculated, despite promises by rich nations. Ensuring the rest have access to vaccines would build trust with the poor world which is lacking, Miliband said, ahead of the vital UN Cop26 climate talks in Glasgow in November.

“We’ve got to deliver on vaccinations to the populations of the world’s poorest countries,” he said in an Observer interview. “It’s a shameful scar on the developing world, what is happening. We promised 870 million doses of vaccine – 100 million have been delivered.”

He said the cost of rich countries providing vaccines, estimated at GBP3bn, was “not astronomical” and was “morally right, it’s doable … even on public health grounds alone, it’s obviously in our interests [because of the rise of variants]. It’s inexplicable and reprehensible to me that this is not happening.”

Miliband, who led the UK delegation at the UN climate talks in Copenhagen in 2009, when he was secretary of state for energy and climate change, warned that the outcome of the Glasgow talks was “perilous – we’re not where we need to be, not by a long way”. He accused Johnson and the Tories of failing as hosts of the summit after a series of missteps. “They have been the most blundering bystanders and they need to stop being blundering bystanders and become credible persuaders.”

The outlook for Cop26 has worsened recently. At the summit, countries must come forward with strengthened obligations to cut greenhouse gas emissions in line with the 2015 Paris agreement ambition of limiting global heating to 1.5C, which scientists say would require a halving of emissions this decade.

But a UN report on Friday showed current pledges from national governments would result in a 16% increase in emissions by 2030. The OECD also published a report, showing that a target of $100bn climate finance – cash from public and private sources provided by the rich world to poor countries, to help them cut emissions and cope with the impacts of extreme weather – would be missed by about $20bn.

Antonio Guterres, the UN secretary-general, warned: “The world is on a catastrophic pathway to 2.7C of heating. There is a high risk of failure of Cop26.”

On Monday, Guterres and Johnson will co-host a meeting in New York, on the sidelines of the UN general assembly, of more than 30 world leaders in a last-ditch attempt to break the impasse with six weeks to go to Cop26.

Miliband warned that rich countries needed to build trust with poor countries, which are most vulnerable to the climate crisis while having done least to cause it. “That’s why Copenhagen failed, because of mistrust between developing and developed countries. Paris [in 2015] succeeded because there was that trust built.”

Miliband accused Johnson and his cabinet of decisions damaging to the UK’s presidency of Cop26, such as slashing overseas aid, dropping the Paris temperature targets from a trade deal with Australia, mooting a new coalmine and failing to kickstart a green recovery from Covid. “They just look like a bunch of greenwashers who want to say they’re committed but then aren’t,” he said.

Alok Sharma, the cabinet minister who will preside over the talks, was “doing a perfectly decent job”, added Miliband, but had been “undermined … totally marginalised and on his own” as no other ministers were taking a visible role.

Miliband, who said the climate crisis was a key reason for his return to frontline politics, also said there was broad cross-party support for Johnson on Cop26, though the prime minister had not reached out to Labour.

“This goes beyond party politics and even who’s in government. It’s much more fundamental than that,” he said. “I’m not saying it [Cop26] is all hopeless and I’m not saying it’s destined to fail. But I am saying, in the strongest terms, the government has got to get its act together and Johnson has got to get his act together. Detail may not be his thing. Focus may not be his thing. But he’s got to summon all he can on this. Because we all need it to succeed.”