A national emergency has been declared in the United Kingdom after a red extreme heat warning was issued for parts of the country for the first time on Friday, reported the BBC.

The red alert was declared for parts of London, Manchester and York for July 18 and July 19 when temperatures could reach record highs of up to 40 degrees Celsius, according to the country’s Meteorological Office.

A red alert is issued when a heatwave is so severe or prolonged that its effects extend outside the health and social care system, according to the weather department.

“Exceptional, perhaps record-breaking, temperatures are likely on Monday, then again on Tuesday,” the weather department said, according to Reuters. “Nights are also likely to be exceptionally warm for the UK, especially in urban areas. This is likely to lead to widespread impacts on people and infrastructure.”

The highest temperature recorded in the UK is in Cambridge on July 25, 2019 at 38.7 degrees Celsius, according to the news agency.

Experts warned that the heat could also lead to deaths of people.

“We could see 1,500 to 2,000 deaths just from this one period of heat,” Bob Ward, the policy director at the Grantham Research Institute on Climate Change and the Environment said, according to The Guardian.

Professor Kevin McConway, emeritus professor of applied statistics at the Open University, also said the country could witness hundreds or thousands of excess deaths from the heat in the coming days.

“It is possible that, because there have been so many warnings about the coming high temperatures, people and businesses will be taking more precautions than usually happen in a heatwave, which could lower the numbers of excess deaths,” he said. “I hope that happens, but I fear there will still be excess death on quite a scale.”

Meteorological Office Chief Executive Officer Penelope Endersby said that the extreme heat forecast was “absolutely unprecedented”.

“People in the UK are used to treating hot weather as a chance to go and play in the sun but this is not that sort of weather,” Endersby said, according to the BBC. She added that the citizens should take the warning as seriously as a red or amber warning for snow or wind.

The forecast has also triggered warnings in London against travelling on Monday and Tuesday. Local government body Transport for London has advised people to go out only for “essential journeys”, according to The Guardian.