Yellow weather warnings for thunderstorms are in force – covering swathes of the north of England, Northern Ireland and Scotland on Sunday – as the summer washout continues.
The warnings mean there is a risk of flooding to homes and businesses as well as the risk of lightning strike damage to properties.
The Met Office also said standing water on the roads and localised flooding could make driving treacherous, with a risk of disruption to public transport.
Northern Ireland expires at 9pm.
It follows a wet Saturday for many places, with parts of London and Glasgow hit with flash floods following frequent heavy rain showers.
A Met Office spokeswoman said although there could be some sunshine around in the eastern parts of England on Sunday morning, it will quickly turn cloudy as rain showers push in from the west.
“Heavy and thundery” showers are expected across much of the UK even if there is no weather warning in place, although there should be glimpses of sunshine in between.
Despite the downpours, it will not feel too chilly, with the mercury forecast to hit the high teens in all four UK nations.
Even though summer has felt like a damp squib, blue skies could be on the horizon later this month, forecasters have said.
Met Office meteorologist Becky Mitchell told the PA news agency: “As we head towards the end of August, there’s a likelihood we’ll see a hotter spell of weather and drier more settled weather across the whole of the UK.”
It might seem hard to believe, but the summer has actually been hotter and drier than average, Ms Mitchell said.
“It’s not been wetter on average but we’ve seen a lot of reports on the news about the flooding. That’s why it may feel like a bad summer, with not much warmth or sunshine,” she said.
Ms Mitchell said the only exception has been the south east of England, which has been wetter than usual and has already had 111% of its average rainfall for the summer.