South set to bask in 24C sunshine
Parts of Britain will enjoy temperatures as hot as Portugal tomorrow as a "tropical continental air mass" moves into the country.
The south east of England is expected to reach up to 24C (75.2F), nearly 10C above the UK's average temperature in May.
However it is set to be cooler across the rest of the country, while a severe weather warning for rain has been issued in Scotland.
The Met Office said: "There's a change to warmer weather for parts of the UK this week due to a tropical continental air mass."
Claire Austin, a forecaster at MeteoGroup, said the warm air from north Africa was in front of a cold weather front, bringing cooler weather away from the South East.
"We're looking at highs of 23C (73.4F), possibly 24C (75.2F) locally, across East Anglia and London," she said.
"Elsewhere, there is a heavy band of rain across Scotland and Northern Ireland that will slowly move south towards England and Wales. The South East should keep its sunny spells."
A severe weather warning for rain has been issued in south west Scotland where a band of rain will move east overnight.
The Met Office chief forecaster said an active cold front moving east across Scotland had brought the potential for 20-40mm of rain within 12 hours, and possibly as much as 50mm in some areas.
The rain will be heaviest across the high ground of eastern Dumfries and Galloway and the southwest of Borders, where some minor flooding is possible, the Met Office said.
Ms Austin said thundery showers were forecast in Scotland tomorrow, with maximum temperatures of 17C (62.6F). Highs of 19C (66.2F) were predicted in northern England, while Wales, South West England and the Midlands could see the mercury rising up to 18C (64.4F).
Wednesday is set to be the "nicest day" for most of the country with dry conditions forecast, before heavy rain is expected on Thursday, she added.
"Tuesday will still be relatively warm but showers are expected," Ms Austin said.
"We might squeeze 20C (68F) in the London area, while the maximum temperature for Wednesday could be 19C (66.2F), before going down to 14C (57.2F) on Thursday."
Portugal's capital Lisbon is also expected to see highs of 24C (75.2F) tomorrow, Ms Austin said.
The average maximum temperature for the UK in May is 14.7C (58.5F), while in England it is 16C (60.8F), according to the Met Office.
Last month saw the UK bask in the sunniest April on record, the Met Office said.
The country enjoyed an average 212 hours of sunshine, provisional figures showed, making it the sunniest April in records dating back to 1929 and beating the previous record set in April 1942 by seven hours.
The highest temperature of the year recorded so far was 25.1C (77.2F) in Faversham, Kent in April, MeteoGroup said.