Forecasters have predicted plunging temperatures across the country even as heavy snow and persistent rain disrupted travellers' return to work after Christmas.
Wales was the worst hit with significant snow in the middle of the country and in the Brecon Beacons in the south.
Some parts of Wales are expected to receive up to 12ins of snow (20-30cm) and drifts later on Wednesday.
Snow showers covered Edinburgh and other parts of Scotland, with Inverness airport remaining closed until 11.30am and train services disrupted in and out of Glasgow Central, Perth and Inverness.
In England, train travel between Bristol Temple Meads and Severn Beach was affected by signalling problems, while passengers in Yorkshire's Howden area were hit by emergency engineering work.
Many planned rail engineering projects across the country upsetted travel, despite Network Rail (NR) assuring there were more trains this holiday period than during last Christmas.
Meanwhile, road accidents in country's various parts led to complete and partial closures of routes.
Stephen Davenport, a forecaster with MeteoGroup, the Press Association's weather division, said: "The same places that got snow today may well get more falls tomorrow. After that, the temperatures will start to fall and the whole country will be back into the cold regime again."
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