The temperature at Heathrow Airport reached 25.8C by mid-afternoon on Monday, and the south is likely to remain warm and sunny until Thursday.
Temperatures are expected to peak on Wednesday, with parts of the south set to reach 28C.
But Sky News weather forecaster Chris England said that the rest of the country may not be basking in the sunshine just yet, with the north expected to be cooler with rain at times.
He added: “Thursday, Friday and Saturday will be cooler overall, then it will turn warm again for most, but the North West will be cooler and cloudier at times, with some rain likely.
“It gets more uncertain going into July, but it looks like central and southeastern parts will be fine, dry and very warm, while the north and west will be more unsettled, with cloud and outbreaks of rain bringing some cooler spells.”
The Met Office said the early indications for July showed the first half of the month bringing a “good deal of dry and settled weather” across the UK.
But forecasters warned that, while the south and east were expected to be warm and sometimes very warm, the north and west would be more likely to be changeable, with showers and possibly persistent rain.
“During these changeable spells temperatures will be close to average for the time of year, but there will be some drier and warmer interludes here too,” they added.
The hottest day of the year so far was 19 April when the Met Office recorded a temperature of 29.1C in St James’ Park, London.