A yellow weather for thunderstorm has been issued by the Met Office as the heatwave is set to come to an end.
The warning is for the whole of Sunday as the Met Office says there is a chance of disruption to parts of England and Wales.
Forecasters said that spray and sudden flooding could lead to difficult driving conditions and some road closures.
There is also a chance that homes and businesses could be flooded quickly, with damage to some buildings from floodwater, lightning strikes, hail or strong winds.
Flooding and heavy rain could also cause delays or cancellations to public transport and power cut, with some communities at risk of being cut off.
The Met Office predicts that the sun remain prominent for most of Friday, but by the afternoon the weather will take a turn for the worse, as showers and heavy bursts of rain threaten to dampen weekend spirits.
Northern Ireland could be amongst the first areas in the UK to experience wet and windy conditions as a weather front pushes in later on Friday, with forecasters saying that around 25mm of rain could fall within the hour in some spots.
The risk of showers moving across to the mainland becomes greater overnight and into Saturday, with forecasters predicting a steady increase in the risk of rain and thunder as the weekend progresses.
But despite the changing conditions, it will remain very warm across the country, with the mercury staying in the mid-to-high 20C for most.
Night temperatures are also expected to stay much warmer than average, providing no relief to those who are most vulnerable to hot conditions.
Met Office Deputy Chief Meteorologist Steven Keates said: "Over the coming days we'll be transitioning to a more unsettled regime for the UK, though temperatures will remain high and it’ll feel very humid for many.
"Heavy showers and thunderstorms are likely to become more frequent through the weekend, with the potential for associated hail, lightning and some gusty winds.
"While the focus of thundery showers on Friday afternoon will be Northern Ireland, that risk spreads more widely across western and southern areas of the UK on Saturday, before pushing further north on Sunday.
"As in many of these situations, these showers can be hit or miss, with some places avoiding them whilst other areas nearby may see some very wet conditions."
The UK Health Security Agency currently has a Heat Health Alert in force. For tips on how to cope with hot weather, you can check out this dedicated NHS advice page.
UK 5 day weather forecast
Today:
Any early morning mist will quickly clear leaving another day with plenty of sunshine for most. Feeling very warm, locally hot. Skies turning hazy across the west with an increasing chance of showers across Northern Ireland, Wales and the southwest.
Tonight:
Most places remaining dry with clear spells, however cloud will continue to build in with showers, locally heavy and thundery developing across western areas, and particularly Northern Ireland. Feeling humid.
Saturday:
A bright but hazy day for many. A continued risk of showers across western areas, and these spreading eastwards across southern England at times. Feeling very warm and humid.
Outlook for Sunday to Tuesday:
Continuing to feel very warm and humid with an increasing chance of thunderstorms almost anywhere throughout the period. Cloudier than recently, but most places seeing decent sunny spells.
