Temperatures could reach a high of 18C in the Capital today, and it is not expected to drop below 7C.
It will stay sunny and clear as the week goes, with expected highs of 14C most days.
Becky Mitchell, senior operational meteorologist at the Met Office, told the PA news agency: "Temperatures are rising quite quickly and there's plenty of sunshine across the whole country today.
"Most places are staying dry and sunny and it looks very similar for much of this week - we hang on to that sunshine."
Ms Mitchell added that temperatures hit 20.5C in Northolt in London, making Tuesday the warmest day of the year so far after Saturday saw 20.2C in north-west Scotland.
But she added: "That obviously happens quite a lot - that through March we get the warmest day of the year so far."
The toastiest day of the current sunny spell will be Wednesday, she added.
However, the warm and dry weather is not expected to last.
On Monday, the sun is expected to leave as temperatures drop in the Capital, and wetter weather may return later in the week.
"As we head into next week, it will probably start off sunny and dry, but we're likely to start to see some showers as the week goes on," Ms Mitchell said.
"It will be turning a little bit more unsettled and also, it will be a little bit colder as well.
"Temperatures are way above average this week but they've going to be a little bit below average by the end of next week," she added.
Ms Mitchell also warned that air quality is "looking quite low", especially in northern and eastern areas of the country as well as towns and cities.
"High levels of air pollution (are) expected in the next few days," she said.
"That's because we've got light south-easterly winds and that's just dragging in pollutants from the continent."