The UK has recorded its hottest day of the year so far after the temperature reached 27.5C (81.5F) at Heathrow on Tuesday afternoon, the Met Office said.
The previous highest temperature was 23.6C in Faversham, Kent, on 6 May.
Average highs for the south of England in May are 17C, although a Met Office spokesperson, Ollie Claydon, said temperatures in the late-20s were not unusual, especially towards the end of the month.
He added the current forecast did not constitute a heatwave, which the Met Office defines as three successive days of temperatures above 28C.
However, not all regions will have blue skies. A band of rain is crossing Northern Ireland, where temperatures will reach about 14C in Belfast, and will become heavier and more widespread across the west and then the north, bringing winds and a cooler feel.
In south-east England, the good weather is expected to continue on Wednesday, with fresher temperatures and showers on Thursday, bringing temperatures down to the early 20s over the weekend.
Most parts of the UK will have above-average temperatures for May over the week but blue skies will be broken up with potentially heavy or thundery showers, especially in the north and west.
Claydon said the outlook for next week remained “borderline” as high pressure was likely to reach the UK over the weekend, but it was unclear whether it would succeed.
Towards the end of the month, the Met Office is predicting lots of fine and dry weather, with near-average temperatures and more summery spells in the east and south-east.
NHS hot weather advice recommends that people stay cool indoors, keep hydrated and out of the sun between 11am and 3pm, apply sunscreen regularly and avoid exercising during the hottest parts of the day.