Much of England and Wales are expected to see temperatures top 30C on Tuesday.
A generally dry day with lots of sunshine will develop into a warm day for many with the heat peaking in southern England and Wales, according to the Met Office.
Chief meteorologist Frank Saunders said large swathes of the UK will feel the heat up to the middle of the week.
While the warmth will carry on for Wednesday, there is a chance of summer thunderstorms in southern areas.
Thursday should develop into a more unsettled day with potentially impactful thunderstorms at times. Friday should bring rain, which will move westward.
The UKHSA warned that expected hot weather may have “significant impacts” on the health and social care sector across the south east and London, with minor impact elsewhere.
The Met Office said a warm snap is considered a heatwave if a location records at least three consecutive days with maximum temperatures exceeding a designated value – 25C for most of the UK, rising to 28C in London and its surrounding area, where temperatures are typically higher.
But while many enjoy the heat, Met Office deputy chief meteorologist David Oliver issued a warning for southern areas to prepare for a wild and wet end to the week.
“There’s a chance of some thundery showers across some southern areas of England on Wednesday, then on Thursday there is a signal for some potentially very heavy thunderstorms to develop,” he said.
“There are still details to confirm during this period, but in any event there is a chance of some impacts on each day, especially Thursday.”
He added: “The heaviest showers on Thursday could result in 20-30mm of rain within an hour, with daily totals possibly reaching as high as 90mm if multiple showers impact the same location. Lightning and hail present additional hazards, with disruption likely for some.
“This is a developing element of the forecast, so it’s important to stay up-to-date with the latest outlook in the coming days.”