Wednesday could be the hottest day of the year and a heatwave could be declared by the end of Tuesday, the Met Office has said.
The forecaster is watching a number of September heat records as temperatures could reach 33C this week, with much of the country seeing temperatures above 30C.
The UK Health Security Agency upgraded its heat alert to amber due to the hot temperatures.
The alerts are in place from 12pm on Tuesday to 9pm on September 10.
Met Office meteorologist Tom Morgan told the PA news agency: “A lot of people will probably think of the summer just gone as being pretty non-exceptional, pretty disappointing if you had plans in the UK.
“This week, it looks likely that we’ll see the highest temperature of the year so far. Today will be the third day that some sites have seen three consecutive days of heatwave conditions.
“By the end of today, officially, it will be a heatwave, it is certain that it will be by tomorrow.”
The forecaster said this week will be the first prolonged spell of hot weather the UK has seen since June following an unusually wet July.
Mr Morgan added: “We may well be close to some record-breaking temperatures in the next few days.
“The daytime maximum temperatures are a little less likely to be broken but nonetheless it will be hot so we are watching a couple of records.
“The most likely record that we could see broken is the highest overnight temperature for Wales. Currently the September highest overnight temperature for Wales stands as 20.5C.
“There is a possibility either tonight or tomorrow night we could see a temperature not fall below that value in parts of Wales.
“The highest UK September temperature we’ve ever seen still stands at 35.6C.
“So we are very unlikely to see temperatures quite that high but probably a 33 is on the cards either tomorrow or maybe also on Thursday, so we are not too far away from the UK record either.”
Temperatures are also set to reach 27C in Northern Ireland and Scotland, the Met Office said.
The temperatures could start to drop gradually at the weekend and there is a chance of thunderstorms breaking out from the north and west of the country, the forecaster added.
By 2.30pm on Tuesday temperatures had hit 30C in London and the Met Office predicted temperatures could reach 31C or 32C later in the day.