The hot weather is forecast to remain across the UK into next week as the NHS warned it contributed to one of the “busiest days ever” in A&E departments.
It comes amid the latest junior doctors’ strike.
The Met Office has forecast temperatures in the high 20s, with some areas reaching into the 30s.
An NHS medical director said the heat had contributed to one of the “busiest days ever” in A&E departments across the country on Thursday.
It comes as the 72-hour junior doctors strike in England, which is currently under way, is due to end on Saturday at 7am.
Earlier this week, the Met Office extended its yellow heat warning until Monday, while several schools in East Sussex closed on Thursday due to water shortages.
It also follows a heatwave which struck swathes of the UK, with temperatures reaching 32.2C in Chertsey, Surrey, on Saturday – marking the hottest day of the year so far.
Dan Stroud, spokesman for the Met Office, said conditions would become “increasingly humid and uncomfortable” over the weekend, especially in inner cities, with temperatures unlikely to drop below the 20s overnight.
NHS national medical director Professor Sir Stephen Powis added: “We know the warm weather has increased demand on services and this week we have seen one of our busiest days ever at A&E departments.
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Strike will ‘make it a bit harder’ to clear NHS backlog “As we move into what’s forecasted to be a very warm weekend, the heat and the impact of industrial action will continue across the country.”
He added that NHS staff are working hard and prioritising urgent and emergency care and urged people to use 999 in emergency and life-threatening situations and NHS 111 online for other health concerns.
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The junior doctors’ walkout comes amid the threat of further strikes throughout the summer if the government does not budge on its pay offer.
The British Medical Association (BMA) said it will ballot training doctors next week to extend its strike mandate.
In April, a similar walkout by junior doctors saw 196,000 hospital appointments and pre-planned operations rescheduled.
The BMA has said junior doctors have suffered a “pay erosion” of 26% over the last 15 years as their wages have failed to keep up with inflation.
They have demanded a 35% pay rise to reverse this.