A severe weather warning, which includes Wiltshire, has been issued for much of England tomorrow.
Some areas of Britain are expected to see nearly half of an entire month's rain fall in just an hour.
The Met Office said thundery rain is expected to start falling early tomorrow morning, with showers unlikely to ease off until the evening.
Some places could see between 0.8in (20mm) and 1.2in (30mm) of rain fall in an hour - almost half of the UK monthly average for the whole of June of 2.9in (73.4mm).
Met Office spokesman Dan Williams said the downpours will be "pretty heavy".
He said: "We are looking at the potential for some localised flooding because they are potentially heavy enough that we could see so much rain fall in a short space of time that it can't drain away fast enough."
The Environment Agency has issued a total of 104 "low risk" flood alerts.
Regions where yellow "be aware" warnings for rain are currently in place are North West England; North East England; Yorkshire & Humber; West Midlands; East Midlands; East of England; South West England; London & South East England; Strathclyde; Central, Tayside and Fife; South West Scotland; Lothian Borders; Northern Ireland; and Wales.
The Environment Agency said: "Heavy rain with isolated torrential downpours may bring some river and surface water flooding on Saturday across all of England, except for Devon and Cornwall, and all of Wales, except for Pembrokeshire."
The predicted deluge is down to warm, humid air moving in from Europe which creates a risk of downpours when it mixes with cooler air over the UK.
Dr Hannah Cloke, a flooding expert, said: "We could see flash floods in some parts of Britain if rainfall this weekend is as intense as has been predicted.
"The problem is likely to be exacerbated in areas where the ground is still wet from the winter floods, particularly in catchments with high groundwater levels and where rivers are still high following Britain's wettest winter ever."
Mr Williams said: "It is going to be one of those warm but humid days where you could see some bright spells but also some heavy showers as well."
He said the downpours will ease by Sunday when there will be some "good, dry and bright spells", particularly in southern and eastern parts of the country and only a risk of light rain in some areas.
Next week is likely to be "slightly unsettled" with some dry and bright conditions along with light showers, he added.
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