A street in York, England, in January. In Britain, about four million people and some $272 billion of assets are at risk of flooding if no action is taken, a report by the agency warnedCredit...Lee Smith/Reuters

LONDON — The British Environment Agency on Wednesday delivered a stark warning to world leaders on the need for decisive action to combat climate change, ahead of a U.N. summit in Glasgow this month when delegates from nearly every country will discuss strategies to tackle global warming.

“It is adapt or die,” Emma Howard Boyd, chairwoman of the government agency, said in a report to the British government.

The recommendations follow a summer of extreme weather in Europe, from deadly fires in Greece to violent floods in Belgium and Germany that experts said were fueled by climate change. With Britain and other countries in the region experiencing more frequent droughts, heat waves and flash floods in recent years, the disasters have driven home the message that a hotter world is affecting wealthier and poorer countries alike and it is only going to get worse.

Lethal floods like those in Germany this summer will happen in Britain sooner or later, no matter how high the country’s flood defenses are built, Ms. Boyd cautioned, urging overarching adaptations to homes, communities and workplaces to make them more resilient to increasingly violent weather.

“While mitigation might save the planet, it is adaptation, preparing for climate shocks, that will save millions of lives,” she said. The report cautioned that environmental regulation had not yet caught up with the changing climate.

Even if countries manage to meet the target of limiting the average temperature rise to two degrees Celsius compared with preindustrial levels — the goal set in the 2015 Paris Agreement — the report said that winter rainfall was still expected to increase by 6 percent and summer rainfall to decrease by 15 percent in the 2050s compared with the last two decades of the previous millennium.

In Britain, about four million people and some 200 billion pounds, or $272 billion, of assets are at risk from flooding caused by global warming if no action is taken, the report warned.

In some cases, access to clean water will grow more difficult in England. Without further action, demand for public water supplies in England will outstrip supply by the 2050s, the report said, exacerbated by droughts and other effects of climate change.

The environment agency said it was working with the government, businesses and communities to prepare and had invested £5.2 billion, or about $7 billion, to shore up flood and coastal defenses over the next six years. It said it had also developed a national framework to manage water supply, and established an $870 million environmental restoration fund.

But with the number of properties built in floodplains in England expected to double by 2065, the agency said it alone cannot protect everyone from increasing flood risks. Instead, it called on communities and businesses to invest in finding ways to live with the risks and to minimize potential damage, such as funding “flood resilience” instead of prevention and restoring wildlife and ecosystems.

Experts said the report served as a reminder that more urgent preparations are needed.

“There remains a gap to implementing measures on the ground. We still don’t see enough being done,” said Lorraine Whitmarsh, professor of environmental psychology at the University of Bath. The inevitability of extreme weather means adaptation measures are just as important as mitigation, she added.

The need for adaptation had already been felt by poorer countries, including some island nations vulnerable to rising sea levels.

Bernard Aryeetey, director of international affairs atWaterAid, an international charity that provides clean water in several countries, said the warning to adapt or die was a reality millions of the world’s poorest people have been facing for decades.

“The alarming news that these devastating impacts will reach our shores must be a rallying call to G-20 leaders to deliver the money for adaptation they have been promising for many years,” Mr. Aryeetey said, calling on Britain to lead the charge.

Dr. Rick Lupton, a lecturer at the University of Bath who has researched climate change mitigation, said cutting emissions remained a critical priority.

“The faster we can cut emissions now, the more we can avoid the worst climate change events in future,” he said.