It comes after thousands of people’s plans to enjoy the long weekend outside were stymied by overcast conditions and the surprise appearance of a haar sea fog.
But forecasters predict long periods of unbroken sunshine in the skies over the Capital on Monday, with the mercury likely to hit 21C by the mid afternoon.
Experts at the Met Office warned people to slap on plenty of sun cream as UV light levels are likely to be high or very high.
The bright weather is expected to last until around 7pm, when temperatures should begin to fall gradually.
Becky Mitchell, a meteorologist from the Met Office, said Monday could be the hottest day of the year for locations across the UK.
“We are expecting highs of 25C (77F) tomorrow, so the warmest day of the year so far, and both today and tomorrow we will see temperatures in the low 20s in most parts of the UK, even in northern Scotland,” she explained.
The previous high of 24.5C (76.1F) was recorded on March 30 at Kew Gardens.
She continued: “The only exception is along the east coast – there is quite a lot of low cloud, mist and fog in east coastal areas and that will stay for the next few days , so highs of between 12C (53.6F) and 16C (60.8F) there – quite a lot lower than elsewhere.”
Ms Mitchell said many places in the South East including London had dawned cloudy on Sunday morning, but that had mostly cleared away by mid-morning.
Although a fine bank holiday is a rarity, in themselves the temperatures are not unusual for the time of year, with the mercury usually sitting in the high teens and low 20s by late May.
Ms Mitchell said: “That could go up to 27C (80.6F) in one or two spots by Wednesday, that is pretty warm for the time of year but not record breaking and not exceptional.”