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.
Weather sayings: True or false?
Weather sayings: True or false?
Seeing a ring or arc around the moon is often a good indication that the weather is changing. The Weather Channel says that due to the structure and angle of an approaching warm front a hazy layer of cloud can sometimes be seen high in the sky before the rain arrives.
Seeing a red sky at night means that an area of high pressure is moving in from the west so there will be a good chance of dry and fine weather the next day. According to The Weather Channel, red sky in the morning means that the high pressure has already passed and wet and windy weather is on its way!
True! A red sky in the morning means the high pressure system has already moved east meaning the good weather has passed and wet and windy low pressure system is heading our way.
Weather records began in 1861 and since then there has been no mention of 40 dry or 40 days of rain after St Swithin's day on 15 July, says The Weather Channel.
The Pennsylvania groundhog (Punxsutawney Phil) gives a weather prediction each year on February 5 and according to tradition, if he sees his shadow and retuns to his hole then there will be another six weeks of winter. But The Weather Channel says Phil's predictions have only been right 39 per cent of the time.
Scientists have proved that there is a link between cows' behaviour and the weather. Researchers found that cows stand when the weather is warmer and are more likely to lie on the ground when it's cooler - such as just before it rains!
Swallows fly at the same height as the insects they are trying to catch and eat. When the weather is warmer, the insects are propelled higher by the rising hot air - therefore the swallows have to fly higher when the weather is warmer!
Late night rain and early morning rain are often an indication of a front passing by and this happens as often during the day as it does as night, which means rain in the morning doesn't mean it won't rain at night.
Seagulls tend to sleep on water but when it's windy and the water becomes choppy they will move inland and huddle on the beach.
This old proverb is thought to be a warning not to take off your clout (winter clothes) until the may blossom (better known as Hawthorn) is out because it heralds warm weather. Until you see it in full bloom there's always a chance the cold weather will return in the spring months, which happens quite frequently in the UK.