The UK Health Security Agency (UKHSA) has issued a yellow heat health alert, meaning vulnerable people are likely to be affected by the weather across most of England.
It will remain in place in eight regions until tomorrow evening (27 June).
NHS national medical director, Professor Sir Stephen Powis, said: “While the warmer weather can lead to additional pressure on services at a time when demand for services is already high."
He added that the upcoming junior doctor strikes are likely to put increased pressure on the NHS as a whole this week.
Monday was the hottest day of the year so far, reaching 28.3C in Wisley, Surrey, and temperatures are expected to rise even higher in parts of central and eastern England on Wednesday, the Met Office said.
Some of the biggest risks to health during extremely hot conditions are heat exhaustion, dehydration and overexposure to UV rays.