BBC Weather's Matt Taylor warned temperatures will plunge to single figures as a huge band of rain blankets the entire UK. Patches of mist and fog in the Midlands and southern England this morning will clear by the afternoon. Temperatures are expected to reach highs of 26C in the southeast before plunging to 14C by Tuesday.
Mr Taylor said: "Over the next couple of days when the sun is out we will see temperatures of around 20C in Scotland and 26C towards the southeast of England but as we go through the week, cold air will be pushed southwards.
"Temperatures in single figures for some, a high of only 14C or 15C in the far south of England.
"It's a few days away but they're the sort of temperatures that we're going to be starting the day with after warm nights.
"This morning, a little bit cooler in one or two spots.
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BBC Weather's Matt Taylor warned tempeatures will plunge to single figures
"Some mist and fog around as well in the Midlands, southern England, parts of Wales and patches through central Scotland.
"A bit more cloud this afternoon across Northern Ireland and Scotland.
"We will start to see more cloud move into the north and west of the country through this evening and overnight.
"The rain will start to get heavier through the highlands and Northern Ireland."
Patches of mist and fog in the Midlands and southern England this morning will clear
People flocked to parks and beaches on Saturday to make the most of the warm and sunny weather, with the mercury reaching 25.2C (77.36F) at Heathrow in the early afternoon.
Sunday is expected to see slightly hotter temperatures, with police officers urging sunseekers not to gather in groups of more than six people.
London and the South East will see the best of the weather, with the mercury hitting just over 26C (78.8F), while Scotland will see 21C (69.8F).
The North West is forecast to see highs of 21C (69.8F) on Sunday, while parts of the North East, such as Newcastle, sitting at around 19C (66.2F).
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John Griffiths, a forecaster at the Met Office, told the PA news agency: "The day will start dry for most areas, with some parts of north east England and Scotland seeing cloud during the morning.
"However as the day breaks it will be hot for many with plenty of sunshine going into the afternoon.
"There could be spells of rain in the south west in places such as Cornwall but they should only be temporary."
Mr Griffiths said this was due to high pressure and "warm air" moving up from southern Europe.