Temperatures could dip as low as minus 10C (14F) in Britain later this week, with snow in some parts, forecasters have warned.
Over the weekend, some areas woke to two icy mornings, with the mercury dipping to a low of minus 1C (28.4F).
Overnight on Sunday, temperatures mostly stayed above freezing, with a touch of frost and sub-zero temperatures in a few isolated locations, such as Cairnwell in the Highlands of Scotland.
Other parts of Scotland, including Braemar and Lerwick and Dun Fell in Cumbria, saw temperatures of 1C-2C above freezing overnight.
In the south, cloud and rain meant temperatures have since risen to 8C-9C (46.4F-48.2F).
Met Office forecaster Helen Roberts said that, despite the mild start to Monday, it will become increasingly cold as the week progresses.
She added: "A return to cold is the theme for this week - cold, breezy with a return of widespread overnight frost and wintry showers predominantly around the coastal fringes of the UK.
"Thursday is looking like it might be the coldest day and night of the week. We will potentially have some lying snow across the hills of Scotland and northern England and that always helps to keep temperatures down, so we could potentially see temperatures as low as minus 8C (17.6F), possibly minus 10C (14F), across parts of Scotland where there is lying snow by Thursday night.
"And even across England and Wales we could see minus 6C (21.2F) to minus 8C (17.6F) and some snow across hillier parts."
She also said there could be ice or snow warnings later in the week.
South Western Railway, meanwhile, warned commuters to check their journey before they travel as the poor weather conditions could cause problems for trains.
On its website, it said: "Due to recent and forecast cold and wet weather, poor rail conditions are expected across the South Western Railway network throughout this week. Ice can cause problems for trains as it can prevent trains from drawing power from the electrified rail.
"Network Rail run special vehicles treat our tracks to reduce the effects of the poor weather conditions and will be taking appropriate action over the next few days.
"We're sorry if your journey is impacted by the weather this week."
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