Downpours are expected to continue throughout the afternoon and evening, with the warning ending at midnight. The warning stretches from the South Coast of England up to Newcastle.
Eastern parts of the country will see the worst of the downpours, with as much as 30mm-40mm in some areas.
Experts believe the deluge could lead to localised flooding and disruption to transport. The Environment Agency has increased its flood risk level from “very low” to “low”.
The Met Office warn that there is “small chance that homes and businesses could be flooded”, and that heavy downpours “could lead to difficult driving conditions and some road closures”.
It adds: “There is a slight chance of power cuts and loss of other services to some homes and businesses.”
A Met Office spokesperson said: “The heavier rain is expected to move eastwards across the day, becoming slow moving across the east later on in the day.
“There is a chance of thunderstorms also and 30-40mm totals over a few hours is possible.
“There is a yellow warning for rain in force from midday to midnight on Sunday, running up the east coast of England. There is a small chance of localised flooding from those heavy showers.”
Saturday will be warm across the South East of England with temperatures peaking at 24C in London. However, there will be patchy showers into the afternoon with South Wales and the Midlands expected to see the worst of the downpours.
Luke Miall, a meteorologist at the Met Office, said: “Showers will continue to drift their way northwards on Saturday evening.
“Otherwise we will continue to see some outbreaks of rain in West Wales, becoming heavier on Saturday evening. For the rest of the country it will be a dry and warm evening.”
In their long-range forecast, the Met Office have warned of a “northwest to southeast” split for the rest of September.
Towards the end of the month, the northwest will see heavy outbreaks of rain, with strong winds and coastal gales also expected. Conditions in the southeast are likely to be drier with temperatures above average.
The weekend’s miserable weather comes just days after a September heatwave saw temperatures soar as high as 29C in London.
Additional reporting by PA
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