As a result, some delegates are now considering sitting out the meeting due to ongoing uncertainties and expected high costs.
"We have people who have registered for the vaccine but the vaccination process (Britain) promised hasn't even started," said Lidy Nacpil, coordinator of the Asian Peoples' Movement on Debt and Development, a regional alliance that promotes climate justice.
A spokesman for the UK's COP26 team said the government was working "tirelessly with our partners, including the UN, to get vaccines to those that need them in time for the summit".
COP26 participants eligible for a vaccine - including government negotiators, civil society activists and journalists - are due to receive an invitation for an appointment shortly, he added by email, without indicating precise dates.
The vaccines that will be offered are those that require a four-week gap between doses, London has indicated.
Climate activists in Africa and Latin America told the Thomson Reuters Foundation the need for a two-week wait after a second dose, to allow time for sufficient Covid-19 protection to build, meant there was no time to lose in starting vaccinations.
Vaccination will not be mandatory for COP26 delegates, but it remains unclear how the UK government plans to ensure the health and safety of attendees if not all are inoculated.
Mohamed Adow, a Nairobi-based African climate expert who has attended every annual U.N. climate conference since 2009, stressed that participants from so-called "red list" countries like Kenya - with high levels of Covid-19 infections - would be required to quarantine in hotels before attending COP26, whether vaccinated or not.
Many developing countries are on the red list for England and Scotland, including the Philippines, Indonesia, Bangladesh, Costa Rica, Argentina, Brazil and Chile.
The British government said in August it would relax some travel restrictions to help delegates attending COP26, including halving the standard quarantine period to five days for those from "red list" countries who have been vaccinated.
But "the costs (of quarantine) are beyond the reach of some poorer governments and smaller civil society organisations", said Adow, who runs an advocacy group called Powershift Africa.
He urged the British government to pay the bill for required quarantine for participants.
The ongoing uncertainty over logistical arrangements for the conference - billed as the last chance to galvanise the action needed to limit global warming to 1.5 degrees Celsius - also threatens to hike travel and accommodation costs, activists said. Costa Rica-based Adrián Martinez, founder and director of La Ruta del Clima, a nonprofit working to expand public awareness on climate change, said it remained difficult for civil society groups to decide which, if any, staff could attend COP26.
That uncertainty was "psychologically... very hard", he said, calling on Britain to clarify the arrangements as soon as possible.
"We haven't had time to actually think about the core issues that we're going to advocate (for at COP26) because we don't know if we're going," he added.
The UN climate change secretariat said it had hoped to receive details of Britain's plans for COP26 by the end of August, but had yet to obtain that information as of Tuesday.
Manila-based Nacpil, who has attended the annual talks since 2007 as an observer, said the challenges related to vaccines, visas and quarantine were discouraging people from regions most at risk from climate change from attending the talks.
"The most vulnerable countries to climate are also the ones that are having all these major Covid-19 problems and are marginalised from vaccine access," she told the Thomson Reuters Foundation by phone. "This is a triple injustice."
Climate groups in developing countries have been discussing in recent days whether to skip the conference entirely, given the health risks and financial costs, she added.
In recent months, British officials have insisted the COP26 summit will be "inclusive" and will push forward work on issues that are a high priority for poorer nations on the frontlines of climate change impacts, including finance and adaptation.
But African expert Adow said that, as things stand, he feared only governments and civil society organisations from rich countries would be able to attend in significant numbers.
"This flies in the face of the principles of the UN process," he said. "A climate summit without the voices of those most affected by climate change is not fit for purpose."