NEW DELHI — John Kerry, the United States envoy for climate change, said Monday that postponing a major United Nations summit on global warming set for November in Scotland would be a “huge, huge mistake.”

“The show must go on,” Mr. Kerry said.

The summit has already been delayed once because of the coronavirus pandemic. Last week a coalition of more than 1,000 environmental groups called for it to be pushed back once again, citing surging caseloads and arguing that attendees from the poorest countries still lack access to vaccines and cannot afford the costs of required quarantining in a hotel.

The Glasgow, Scotland, summit, known as COP26, is being billed as the planet’s last best chance to curb the fossil fuel emissions that are driving climate change. About 20,000 diplomats, business leaders and activists are expected to attend and nations are under growing pressure to announce ambitious action to avert the worst consequences of climate change.

Mohamed Adow, director of Power Shift Africa, a research institute in Kenya and one of the groups calling for postponement, said he fears “it is only the rich countries and N.G.O.s from those countries that would be able to attend.”

But a group of 39 small island nations, which are among the most vulnerable to climate change, are insisting that the global gathering should take place as planned. A delay would only benefit the fossil fuel industry, the group said in a statement.

The British government said last week that the first doses of the AstraZeneca vaccine would be administered to every delegate who requests one. Alok Sharma, the United Nations president of the conference, said in a statement that Britain also intends to cover the full costs of quarantines for delegates from its “red list,” who otherwise would be barred from entry because of the high rate of Covid-19 cases.

Mr. Kerry said he believes the conference can be done safely.

“There is no reason to delay in this world of vaccines and social distancing,” Mr. Kerry said. And he warned that “any delay will allow nations to backtrack, to backpedal, maybe renege on a commitment.”

The latest scientific assessment from the United Nations scientific panel found devastating impacts of global warming are now unavoidable and the window for averting even worse consequences is closing fast.

Mr. Kerry said the warning from scientists underscores the urgency for the Glasgow gathering.

“We don’t have time to mess around with reconvening,” Mr. Kerry said. “The world needs to get cooking on this.”