Temperatures are set to drop by as much as 10 degrees in less than a week, meteorologists have said.
An Indian summer is expected to arrive on Tuesday, with temperatures climbing to a maximum of 20C (68F) in London.
But it will only last a day, as northern winds bring a chill to the UK which has so far seen mild autumn temperatures.
The sunshine and warmth will give way to more unsettled weather as temperatures drop by a few degrees every day to 8-9C (46.4F) on Friday.
Sky News weather presenter Nazaneen Ghaffar said: "For the first part of this week, we'll experience rather mild conditions, especially across southern parts of the UK. This is because of the warm air being drawn up from the near continents in the south."
However, we should not expect wall-to-wall sunshine, Ghaffar said.
"For England, Wales and much of Ireland, there will be a warm front bringing light and patchy rain or drizzle and also a lot of cloud," she said.
"The best of the sunshine will be across northern Scotland, but it will still be cooler than the South.
"However, it does look like we could have some sunny spells in southeast England, and as a result this could boost temperatures up to 20C (68F)."
But Ghaffar said: "By Thursday, a cold front slips south from the north of the UK and the winds switch to a fresh northerly direction."
"This will introduce much colder air across all of the UK and Ireland by the end of the week. As a result, temperatures will be around 7-9C.
"There also looks to be some wintry showers across the far north and east."
Meanwhile, early-morning fog has disrupted flights at airports in the UK today.
London City Airport experienced delays while there were 30-minute hold-ups for both incoming and departing flights at Heathrow in west London.
London City said: "Due to low visibility this morning, flights to/from London City Airport are experiencing disruptions.
"Passengers are advised to contact their airlines for more information regarding their flight status."
A Heathrow spokeswoman said: "We're hoping that the visibility will improve soon."
There were also delays and some cancellations to flights at Newcastle Airport due to fog.
The unseasonable weather follows a year that has seen some of the biggest fluctuations in rainfall on record.
A drought affected large swathes of England during the spring, prompting a number of hosepipe bans.
But that gave way to weeks of torrential downpours in some regions, with a series of floods during the summer months.
