The alert, issued by the UK Health Security Agency, runs from 9pm Thursday (June 12) to 8am Sunday (June 15) and covering the east of England, East Midlands, London, and the south east.
Oxford is set to hit 26°C on Friday (June 13) while some parts of the UK could be hotter than Ibiza, Mykonos, and even Los Angeles.
READ MORE: High use of new skate park near Oxford ‘heart-warming to see’
Met Office forecaster Simon Partridge said: “We’ve got a very warm, humid, spell of wet weather to come.
“It’s all because we’ve got air moving in from the south, so the air is originating across Spain and Portugal and has been moving its way northwards.
“We’re going to see increasingly humid conditions with very warm days and some quite muggy nights as well and the general gist is that western parts of the UK are likely to see the majority of the rain and the thundery showers.
READ MORE: New coffee shop to open soon on busy city road
“But, as we go into Friday, we could see some of that also moving across the east and south east as well for some thunder there for a time.”
Under the UK Health Security Agency and the Met Office’s Weather-Health alerting system, a yellow alert means there could be an increased use of healthcare services by vulnerable people.
It may lead to an increase in risk to health for individuals aged over 65 or those with pre-existing health conditions, including respiratory and cardiovascular diseases.