The all-time record temperature for February could be broken this week, with highs of 20C possible.
Monday is set to see the mercury hit 19C for the second consecutive day, with even higher temperatures on the way.
The Met Office said a record-breaking 20C may be felt in north-west of England and London on Tuesday.
Londoners were seen enjoying the sun in the city’s parks yesterday, with Hampton Water Works hitting the highest temperature in England at 19C.
Families were spotted basking in the sun in Dorset, with keen swimmers taking a dip in the sea in Bournemouth.
Met Office meteorologist Helen Roberts said: “This morning there will be some fog patches, particularly in the north east of England and Welsh borders, but that will lift and clear readily.
“It’s going to be a really decent day with lots of sunshine, apart from in northern Scotland where we’ll see some cloud and outbreaks of rain.
“It’ll be around 18-19C today in the North West and the London area – it’s going to be a lovely day and exceptionally mild for this time of year.
“On Tuesday it’s a similar picture but with more sunshine, it could hit 20C which would break the all-time record for February, that’s not out of the question.”
The temperature might have been affected by a phenomenon known as the Foehn Effect, a dry and warm down-slope wind that occurs on the downside hills or mountains.
Temperatures across Wales reached 18C in many places on Sunday, almost hitting the previous February record of 18.6C, set in the south Wales village of Velindre in 1990.
But the good weather is set to dissipate towards the end of the week, with wind and cloud forecast for the weekend.
Ms Roberts added: “On Wednesday, we’ll have a foggy morning which will be slower to clear and there’ll be more cloud throughout the day with less sunshine.
“There’ll be a definite change on Thursday, with the possibility of heavy showers everywhere.
“That sets up a change of unsettled weather into the weekend. Temperatures will be down, although a tiny bit above average still, but the winds and cloud will make it feel very different to the early part of the week.
“This will continue into next week.”
