A global network of more than 1,500 climate organisations called on Britain to postpone November's Cop26 summit, saying on Tuesday that a lack of Covid-19 vaccines could sideline developing countries.
An increase in Covid cases, unequal global vaccine distribution and stringent quarantine requirements for more than 60 "red list" nations or territories hoping to attend the 12-day UN talks mean that "a safe, inclusive and just global climate conference is impossible", the Climate Action Network said.
"Our concern is that those countries most deeply affected by the climate crisis and suffering from the lack of support by rich nations in providing vaccines will be left out," said Tasneem Essop, the network's executive director.
"There has always been an inherent power imbalance within the UN climate talks and this is now compounded by the health crisis."
But the UN rejected the call to delay the Cop26 summit, which is set to begin on November 1 in Glasgow, Scotland.
"For now, no changes are planned but we understand the concerns," UN deputy spokesman Farhan Haq said.
Host government Britain also opposed a delay, saying the climate crisis was too urgent.
A recently released UN climate science report shows "why Cop26 must go ahead this November to allow world leaders to come together and set out decisive commitments to tackle climate change", summit president Alok Sharma told AFP.
Mr Sharma said the conference had already been postponed once because of the pandemic. It was supposed to be held last November
"The global scientific community has made it clear that climate change is now a global emergency," Mr Haq said.
"Only an urgent and major step up in climate action can keep the goals of the Paris Agreement within reach, and protect the most vulnerable countries and communities from worsening climate impacts."
He said UN authorities were working with London "to make Cop as safe and inclusive as possible, including offering vaccines to all participants and paying for hotel quarantine costs where quarantine is required".
The northern hemisphere has been battered over the past three months by record-breaking extreme weather made worse by global warming, scientists say.
There have been deadly heatwaves in parts of North America and Europe, unprecedented flooding across Western Europe, China and the US, and uncontrolled wildfires around the Mediterranean basin and in California.
Britain has said it will cover accommodation costs for delegates subject to the quarantines, and has offered to provide vaccines.
"We are working tirelessly with all our partners, including the Scottish Government and the UN, to ensure an inclusive, accessible and safe summit in Glasgow," Mr Sharma said.
But delegates who have applied for them have yet to receive their shots, the network said.
The British government said the vaccinations would start this week, and that even with a four-week delay between doses there was still enough time to complete inoculations before October 31.
More than 55 per cent of Europeans are fully vaccinated, compared to about 3 per cent in Africa.
Civil activists, who play a crucial watchdog role as registered observers, will also probably face restricted access, the network said.
Developing countries will be deeply affected by decisions made at the summit on issues including climate finance, international carbon markets, and how to help poor nations cope with severe climate damage already incurred.
"A climate summit without the voices of those most affected by climate change is not fit for purpose," said Mohamed Adow, director of the think tank Power Shift Africa, in Nairobi.
"If Cop26 goes ahead as currently planned, I fear it is only the rich countries and NGOs from those countries that would be able to attend.
"This flies in the face of the principles of the UN process and opens the door for a rich nations stitch-up of the talks."
The network said the call to delay Cop26 should not be seen as a boycott.
"We will continue our work to push political leaders to deliver ambitious national climate targets, fulfil their responsibilities on climate finance and phase out fossil fuels," it said.