Lower temperatures, rain and strong winds are all predicted for the next few days.
Britons have been making the most of the balmy weather with the mercury hitting 25C on Monday but this is set to come to an end.
Temperatures are set to plunge by up to 10C and large swathes of the country can also expect some rain by Wednesday, the Met Office said.
There will also be some reasonably strong winds in the North West.
Met Office operational meteorologist Martin Bowles said: ‘We are saying there could be a 10C change in the maximum temperature.
‘We have had this period of rather summery weather over the last few days – that is going to change on Tuesday and Wednesday, quite significantly.’
It comes as warm air from southern Europe is replaced by cold air from the north Atlantic which should see temperatures drop by Thursday.
It is still going to be warm, dry and sunny in parts of the UK on Tuesday – in areas such as south-east England, the Midlands and East Anglia – but cloud is set to build up in the North West.
The Met Office said it may still reach 26C in some places on Tuesday, including in London, but 16C is set to be the highest temperature anywhere in the UK by Thursday.
Mr Bowles added: ‘There will be a very autumnal feel to the weather which is quite appropriate in a way because Tuesday is the autumnal equinox.
‘It is when the astronomical autumn begins so it is quite appropriate we are getting quite a significant change in weather.’
He said: ‘You would expect a gradual change as we move later in the year, it just happens that as we have got these low pressure systems coming in tomorrow that it coincides with the autumnal equinox.’
In a forecast covering Wednesday to Friday, the Met Office said: ‘Turning much colder than recently and unsettled with showers and occasional longer spells of rain.
‘Some drier, brighter interludes are also likely, especially in the west on Friday.’
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