Although the hot weather isn't expected to last long, temperatures are predicted to soar to 27 degrees in Milton Keynes.
England and Wales are expected to experience warmer weather from Sunday, with temperatures rising to around 27C in Milton Keynes on Wednesday, the Met Office said.
But the hot spell will come to an abrupt end with rain in the second half of the week.
People are being warned that while the peak of summer has passed, they could still suffer sunburn if they fail to take precautions.
Met Office meteorologist Craig Snell said: "The sun is still fairly strong so if you're going to be in the sun for kind of a good length of time, wear a hat and some sun cream because even though it would be kind of past the peak of the summer, the sun can still burn you at this time of year."
Mr Snell said temperatures are likely to be higher than expected for this time of year.
"For some of us I think they will probably be a good five degrees above average for the time of year," he said.
"At the early part of September we're probably looking at an average of about 21C (70F) across southern parts of the UK.
"In the North, we are probably looking more around the high teens so 17-18C (63F-64F)."
Mr Snell also said: "September over the last decade or so, we have had a warm spell especially towards the beginning of the month."
