Weather warnings have been issued for heavy showers, thunderstorms and a small chance of flash floods posing a "danger to life".
The Met Office issued three yellow warnings for thunderstorms across England, Scotland and Northern Ireland on Tuesday, stretching from midday up until the evening.
In the warnings, the weather agency said there is "a small chance of fast flowing or deep floodwater causing danger to life".
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The east of Northern Ireland, southernmost parts of Scotland, and parts of northern England are set to be worst affected, where the Met Office say "20 to 30 mm of rain may build up" over the evening.
It added: "Frequent lightning, hail and gusty winds will be additional hazards in some places."
Meanwhile, from Yorkshire to the east of England, the Met Office said "thunderstorms will break out in some places this afternoon and move quite quickly eastwards" before fading later.
"Where storms do occur, 15 to 30 mm of rain may fall in less than an hour or two in a few spots and frequent lightning, hail and gusty winds will be additional hazards," it added.
Another yellow thunderstorm warning is also in effect in Scotland on Wednesday, lasting from 10am to 7pm, and stretching from Edinburgh to Inverness.
The Met Office said showers on Wednesday in Scotland "will become heavier and more widespread later in the morning for the afternoon".
"Where showers and storms develop, they could be slow-moving and prolonged, bringing 15-20 mm of rain in less than an hour and potentially 30-40 mm in a few hours," it added.
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Tap hereThe weather agency said "sunshine and showers" are set for Thursday, with some rain being "heavy with hail and thundery".
It then says in its weekly outlook that the weather will "slowly becoming drier through Friday and into Saturday, with increasing amounts of sunshine".
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