Thursday is expected to be the warmest day, with a mixture of sunshine, rain and cloud as the mercury rises between Monday and Wednesday, the Met Office has said.
However, temperatures are unlikely to surpass the 23.4C (74.12F) recorded at St James’s Park in London last month, which was the hottest day of the year.
Met Office forecaster Simon Partridge told the PA news agency: “This week is a funny old one, there’s a fair bit of rain and cloud around as well as sunny spells, with many areas gradually warming up until Thursday which should be the warmest day.
“Between Monday and Wednesday temperatures will sit in the high teens and low 20s in Wales and central and southern parts of the England, while eastern Scotland could reach 19C (66.2F).
“But in parts of Northern Ireland, northern England and Scotland there will be cloud and some scattered showers.
“Thursday will see high teens quite widely across the country, parts of the South East will rise as high as 21C (69.8F), with an outside chance of 22C (71.6F) in areas where there is more sunshine.
“So there’s a warm spell in store for many but don’t expect the wall-to-wall sunshine and blue skies we saw in April.”