Saturday was the hottest day of the year so far, and temperatures are expected to soar as the heatwave continues into next week.
Clear skies and strong sunshine have been enjoyed by the majority of Britons.
Temperatures are expected to continue rising into the low 30s into early next week.
Met Office forecaster Charles Powell said: "We have seen the hottest day of the year so far.
"Sunday and Monday will see repeat performances of Saturday, with a lot of England and Wales seeing dry, sunny, warm, humid weather with high UV, high pollen.
"All throughout this period we will have some pretty mild uncomfortable nights - a lot of places in the mid to high teens for night-time temperatures."
Some parts of the UK will see temperatures 10C above their averages for this time of year.
"The sun is as powerful as it can be," said Mr Powell.
"I would say we are in the midst of a heatwave. You can't really deny the fact that most places are well above average in terms of temperatures, and it feels pretty warm."
Public Health England (PHE) has issued a heat health warning.
Dr Thomas Waite from PHE said: "For some people, such as older people, those with underlying health conditions and those with young children, the summer heat can bring real health risks.
"That's why we're urging everyone to keep an eye on those you know who may be at risk this summer. If you're able, ask if your friends, family or neighbours need any support."
Unusual levels of UV are also being recorded in the UK at the moment - with the strength of the UV in some spots being as high as that in Cyprus and Gibraltar.
This has prompted warnings that people should take extra care in the sun.
Full Met Office forecast for South Yorkshire:
It will be another fine day with almost unbroken sunshine and light winds. It will become hot inland, although perhaps a little cooler near the coast with a sea breeze developing. Maximum temperature 30C.
It will be a fine end to the day with plenty of evening sunshine. Overnight, it will remain warm and muggy. Minimum temperature 16C.
It will be another hot day with good spells of sunshine. Cloud may build a little during the afternoon, with the chance of the odd shower later. Maximum temperature 31C.
Outlook for Tuesday to Thursday:
It will feel significantly fresher on Tuesday and Wednesday with variable cloud and sunny spells. Thunderstorms are possible overnight on Wednesday, these clearing on Thursday to sunny spells.