Britain is braced for two days of thunderstorms which could bring hail, lightning and more flooding.
The alert covers a large part of southern and eastern England, stretching from Bath in the southwest to Hull in the north.
Heavy downpours could cause particular disruption to already rain-hit places such as Lincolnshire, where almost 600 households in and around Wainfleet were told to leave as the area was hit by flash flooding.
The town first flooded on Wednesday after more than two months’ worth of rain fell in two days, causing the River Steeping to burst its banks.
The Environment Agency described the situation as “unprecedented” after 132mm of rain fell between Monday and Wednesday.
RAF Chinook helicopters dropped an additional 76 tons of sand and ballast in the area on Sunday after drafting in 270 one-ton sandbags in an attempt to repair the bank on Friday.
Forecaster Dean Hall said the Met Office was “keeping an eye on it as there could be some issues with surface water and flooding”.
There were six flood warnings and 17 flood alerts in place on Monday morning, affecting parts of eastern and western England.
The northeast, Scotland and Northern Ireland are expected to see periods of heavy rain and hail on Monday, broken up by bright, sunny spells.
Rain will also hit parts of northwest England, the Midlands and Wales, but will die out by the evening, forecasters say.
In the southeast, a cloudy start to the week is expected, while temperatures in the region, East Anglia and along the south coast are predicted to hit around 21C and 22C.
Things are then expected to take a turn for the worse on Tuesday and Wednesday.
Sunny spells and showers are predicted for Thursday and Friday, with the heaviest rainfall in the north.
Forecaster Bonnie Diamond told The Independent: “On Tuesday temperatures will be in the low twenties in southern England with a chance of 24C in the best of the sunshine.
“Quite a wide area could see showers and thunderstorms on Tuesday and Wednesday.
“With the heaviest showers there’s a chance 15-30mm could fall within an hour, and with that there’s a chance of hail.
“It looks like it will be fine and dry this weekend but next week will be more unsettled, particularly for northwestern parts of the UK, although it could stay drier and warm in the southeast.”
Agencies contributed to this report