Green groups say that the COP26 climate conference due to be held in Glasgow in November should be postponed.
They argue that vaccine inequity and unaffordable accommodation will lock out "huge numbers" of developing country delegates.
But the UK government now says it will fund quarantine hotel stays for delegates, observers and media arriving from red list countries.
Vaccines are being rolled out for any delegate who needs one, ministers say.
The COP26 negotiations are seen as the most crucial gathering on climate change since the Paris agreement came into being in the French capital in 2015.
Around 200 heads of state and government are expected to attend, with thousands of delegates, civil society members and media.
Environmental groups are an important element in these global talks process.
While they have no direct power to influence outcomes, they act as observers and as advisers to many poorer countries that have limited personnel and resources to cover every aspect of this sprawling negotiation.
The Climate Action Network represents more than 1500 civil society organisations in over 130 countries.
They have been concerned for some time that the global response to Covid-19 was likely to impact delegates, campaigners and journalists from Global South countries, many of which are on the UK's red list for the virus.
In a statement, they point to the fact that, according to the WHO, around 57% of people in Europe are now fully vaccinated, while in Africa the figure is around 3%.
"Our concern is that those countries most deeply affected by the climate crisis and those countries suffering from the lack of support by rich nations in providing vaccines will be left out and be conspicuous by their absence at COP26," said Tasneem Essop, executive director of Climate Action Network.
"There has always been an inherent power imbalance within the UN climate talks and this is now compounded by the health crisis. Looking at the current timeline for COP26, it is difficult to imagine there can be fair participation from the Global South under safe conditions and it should therefore be postponed," she added.
While the UK has repeatedly said that vaccines will be made available for any delegate who needs them, environmentalists point out that attendance at COP26 is about far more than just access to a jab.