High temperatures and sunshine are predicted this week in Merseyside, with highs of 27C expected.
Despite the official meteorological end of summer, warm weather is expected to last for several days, with the mercury hovering around the mid-20s in many parts of the country.
But the Met Office said it is still "touch and go" whether some areas will tip into official heatwaves.
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Liverpool saw highs of 23C on Monday while 27C is expected Tuesday, warmer than the likes of Faro in Portugal.
The warm weather follows the official end of summer, from a meteorological point of view, which is considered to be August 31.
The Met Office said the warm spell is expected to last until Wednesday, and predictions show some areas are on the "borderline" of experiencing an official heatwave.
A location meets the UK heatwave threshold when it records a period of at least three consecutive days with daily maximum temperature levels which vary across the country.
Met Office spokesman Stephen Dixon said: "It's not uncommon for the UK to get warm spells heading into early September.
"There's a chance of some areas reaching heatwave criteria, but that is fairly borderline and the breakdown that is happening on Wednesday will subdue the temperatures as well.
"So it will be touch and go for some places whether a heatwave is officially in the forecast."
He added: "Temperatures (on Monday) are going to be widely in the mid-20s, especially in the south, where there's going to be spells of good sunshine.
"Heading into Tuesday, that warmth spreads further north across the country, bringing a fine and dry day for most with that warm weather extending across the UK."
The mercury is expected to exceed the average for September, which is 18C (64.4F) in the UK.
Hour by hour weather forecast for Tuesday, September 7 in Liverpool:
6am: Clear sky, 16C
7am: Sunny, 16C
8am: Sunny, 16C
9am:Sunny, 17C
10am: Sunny, 18C
11am:Sunny, 20C
12pm: Sunny, 22C
1pm: Sunny, 24C
2pm: Sunny, 25C
3pm: Sunny, 26C
4pm: Sunny, 27C
5pm: Sunny, 27C
6pm: Sunny, 27C
7pm: Sunny, 26C
8pm: Clear sky, 25C
9pm: Clear sky, 24C
10pm: Clear sky, 23C
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