Wednesday could be the hottest day of the year and a heatwave could be declared by the end of Tuesday, the Met Office said.
The forecaster is eyeing a string of September heat records as temperatures could reach 33C this week, with much of the country experiencing temperatures above 30C.
The UK's Health Safety Agency has upgraded its heat alert to orange due to the scorching temperatures.
The warning covers all areas of England except the North East, where there is a yellow alert.
The notices are in effect from 12 p.m. on Tuesday until 21:00 on September 10.
Met Office meteorologist Tom Morgan told the PA news agency: “A lot of people will probably think that the summer that has just passed has been quite unexceptional, quite disappointing if you had plans in the UK.
“This week, it looks likely that we will see the hottest temperature of the year so far. Today will be the third day that some locations have seen three consecutive days of heat.
“By the end of today, officially, it's going to be a heatwave, it's sure to be until tomorrow.”
The forecaster said this week will be the first prolonged heatwave the UK has seen since June after an unusually wet July.
Mr Morgan added: “We may well be close to some record temperatures over the next few days.
“Daytime highs are a little less likely to break, but it will still be warm, so we're eyeing some records.
“The most likely record we could see broken is the highest overnight temperature for Wales. Currently, the overnight high for Wales is 20.5C.
“There is a chance that either tonight or tomorrow night we will see a temperature not drop below that value in parts of Wales.
“The highest September temperature ever seen in the UK remains at 35.6C.
“So it's very unlikely we'll see temperatures that high, but probably 33 is on the cards either tomorrow or maybe even Thursday, so we're not too far away from the UK record.”
Temperatures are also expected to reach 27C in Northern Ireland and Scotland, the Met Office said.
Temperatures may start to gradually drop over the weekend and there is a chance of thunderstorms from the north and west of the country, the forecaster added.
By 2.30pm on Tuesday temperatures had reached 30C in London and the Met Office predicted temperatures could reach 31 or 32C later in the day.