The Met Office has expanded its extreme heat warning for the UK, predicting record-breaking highs of 38C (100.4F) this week.

The Met Office forecasts that extremely high temperatures could last from Monday until Thursday, leading to health concerns for elderly and vulnerable people. The forecaster said there was “growing confidence” that this week may break the record for the hottest June temperature of 35.6C, which was set in 1976 in Southampton and Camden Square, London, in June 1957.

“The forecast heatwave is developing into an impactful severe weather event, with record-breaking June temperatures and very high humidity,” said Tom Crabtree, a Met Office deputy chief forecaster. “The combination of heat and humidity will be oppressive and bring impacts across society from public health and infrastructure, to power and water supplies.”

Overnight temperatures will also be very high, with widespread tropical nights, where the temperature does not drop below 20C, across southern parts of England and especially in urban areas. Humidity is also a factor, making this heatwave even more impactful with heat stress a danger to all.

“This will make it very hard for people to recover from the daytime heat, exacerbating the heat stress impacts,” said Crabtree. An increase in potential water safety incidents is also noted, as more people are expected to visit coastal areas, lakes or rivers.

Monday and Tuesday’s warning covers most of southern England, central and eastern Wales, and much of the Midlands. The peak of the heatwave is forecast for Wednesday and Thursday, when temperatures are expected to climb to 38C, and when the Met Office alert extends to parts of Greater Manchester, Yorkshire and Cheshire. By Friday, conditions will be less hot, with highs of 33C forecast across eastern areas.

People in the alert area are advised to drink plenty of fluids, keep out of the sun and avoid exercising between 11am and 3pm, the Met Office said.

The UK Health Security Agency (UKHSA) has also issued a series of yellow and amber heat health alerts, warning that health and social care services must ensure they are prepared.

Dr Agostinho Sousa, the head of extreme events and health protection at UKHSA, said: “Sustained periods of warm weather can result in serious health outcomes, especially for older adults, and it is therefore important that everyone takes sensible precautions while enjoying the sun.

“Simple steps such as staying hydrated, avoiding the sun during the hottest part of the day, and keeping your home cool are some of the key ways to help keep yourself and others safe during periods of hot weather.”