In a statement, the UKHSA warned that cold weather could have “serious consequences” for health and urged Britons to look out for those at a higher risk.
The mercury will drop to a low of 2C in the early hours of Monday morning, according to BBC Weather. It will fall as low as 1C on Tuesday evening.
It follows a warmer Saturday afternoon which will see temperatures in the capital climb to 11C.
Dr Agostinho Sousa, Consultant in Public Health Medicine at UKHSA, said: “Cold weather can have serious consequences for health, with older people and those with heart or lung conditions particularly at risk.
“It’s important to check in on family, friends and relatives who are more vulnerable to the cold weather. If you have a pre-existing medical condition or are over the age of 65, it is important to try and heat your home to at least 18°C if you can.’’
David Oliver, Deputy Chief forecaster at the Met Office, said: “From Sunday and into early next week an area of high pressure will dominate the UK’s weather. This will bring some cold nights with a widespread frost across the country. However, by day temperatures will recover to around mid-single figures, near normal for the time of year.”
Mayor Sadiq Khan last month issued a high air pollution alert for London as temperatures plunged as low as -2C.
Cold, still and foggy weather can increase pollution levels as it makes it harder to dissipate car fumes.
Frank Kelly, professor of community health and policy at Imperial College London, warned that anyone with respiratory conditions such as asthma and chronic obstructive pulmonary disease (COPD) was at particular risk from toxic air.
Air pollution is estimated to contribute to up to 36,000 early deaths in Britain every year, according to Asthma + Lung.