Weather forecasters fear the recent sunny weather is about to be replaced by storms during the remainder of the bank holiday weekend.
While the North East was hit by minor showers on Saturday following a prolonged warm spell, more sustained rainfall is possible both Sunday night and throughout Monday.
The Met Office fears the change could even trigger flash flooding with both the south of England and Wales likely to bear the brunt.
Its "yellow" warning, covering between midday on Sunday and 4pm on Monday, warned homes and roads could fall victim to the turn in conditions.
Heavy downpours could feature with thunder and lightening although the "hit and miss" nature of storms meant "many places will likely see little or no impacts", the update said.
It will dampen the summertime mood many enjoyed this weekend, as blue skies and high temperatures prevailed over much of the country.
On Saturday, the warm conditions peaked at 27 C (80.6F) in Weybourne, Norfolk, and Sunday saw temperatures in the low 20s, the Met Office said
The heat comfortably outstripped the average for late May, a spokesman added, which tends to linger between 16C (60.8F) and 18C (64.4F) in England and Wales.
The Met Office weather warning said: "Some areas of heavy rain will move north-eastwards during Sunday and into Monday across parts of southern England and Wales with some locally torrential downpours and thunder possible.
"Many areas will see rain at times but the extent of the most intense and thundery downpours will be limited. Only small areas are likely to see these downpours at any one time.
"This could result in some disruption, with sudden flooding of roads, transport routes, homes and businesses possible locally. Frequent lightning in association with the heaviest rain may also temporarily disrupt power supplies.
"However, given the hit-and-miss nature of the heavy rain and thunderstorms, many places will likely see little or no impacts."
Average North-East temperatures are expected to drop from 19C on Sunday to 12C on Monday with the sunniest day of the new week likely to be Wednesday with temperatures rising back to around 20C.