Temperatures on Tuesday are expected to peak between 30C and 32C in the South East, while Scots can expect an above-average 20C-21C, the Met Office said.
It means Britain could be as warm as Bangkok in Thailand, and hotter than predictions for Madrid and Los Angeles.
However, gale force winds are forecast to hit the west coast of Scotland and Northern Ireland by the end of Sunday ahead of the warmer air arriving.
The last time temperatures soared above 30C in September was in 2006 in Kew Gardens, which hit 30.5C on September 11.
If the mercury rises above 31.6C, which was reached at Gatwick on September 2 1961, then it will be the hottest day for 55 years.
Met Office forecaster Simon Partridge said: “Basically we’ve got air coming up from the south. The origins of this air is generally southern France and northern Spain, where things are fairly warm at this time of the year. So we’ll start to see things warming up.”
The highest September temperature recorded was in 1906 when the mercury hit 35.6C in Bawtry, South Yorkshire.
Most of England will bask in temperatures in the high 20s, but it is likely to rain in western Scotland and Northern Ireland, which could also spread to south-west England and western Wales.
Britons can expect to bake in above-average temperatures across the UK for the rest of the week, the Met Office said.
Conditions will become more unsettled on Thursday and Friday when a band of rain will sweep east, bringing scattered showers.
The week after next will split the UK, with the North West experiencing bands of rain interspersed with dry spells, while higher pressure over Europe will hold on close to the South East, bringing drier, warmer and more settled conditions.