Temperatures will remain above average for the next few days before dropping at the weekend.
Experts warn Britons to expect morning frosts and fog as cold air meets warmer conditions currently over the UK.
Thermometers will hit 55F (13C) in the South through this week with the North cooler between 46F) and 50F.
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But by the weekend the mercury will struggle to get out of single figures in the South, with parts of Scotland dropping close to freezing.
It is a mixed week ahead, with rain affecting western parts of the UK
Met Office forecaster Oli Claydon said: “Temperatures will return to more average figures for the time of year by the end of the week.
“Most places will stay in double figures through the next few days, before it turns cooler on Saturday. There might be some rain around, although for most of the country it is gong to remain relatively calm and settled.”
With 12 days to go, weathermen are scratching their heads over whether Britain will see a white Christmas.
Met Office spokeswoman Nicola Maxey said: “There is a large degree of uncertainty as we head towards the Christmas period.
“However, high pressure currently looks more likely to dominate, leading to drier weather, but it will still be rather cloudy with some fog patches.”
WeatherOnline forecaster Simon Keeling said that, after a damp and grey week, it will turn cooler by the weekend.
He said: “It is a mixed week ahead, with rain affecting western parts of the UK.
“There will be a tendency for temperatures to cool towards the weekend and into next week, with fog becoming more widespread, and a risk of frost too.”