Freezing temperatures mean the day is on track to be the coldest March day since 1986, forecasters have said.
The unseasonably cold weather left travellers battling ice and snow, with the South of England being worst hit.
Jersey Airport was shut due to snow, and on the roads there were hazardous conditions on many major routes.
Blizzards were reported on the M40 in Warwickshire and there were numerous rush-hour accidents.
Strong winds added to drivers' problems with falling trees blocking some roads, while on the trains poor rail conditions led to delays in Sussex and Hampshire.
Forecasters warned of snow spreading to nearly every area of Britain, with almost 4in (10cm) expected in the worst-hit areas. Although too early to officially record, it is thought to be the coldest at this time of year in 27 years.
The last time the UK experienced such a cold March day was on March 1 1986, said Laura Caldwell, a forecaster with MeteoGroup. In contrast, this time two years ago the country was basking in balmy temperatures of up to 17C (63F).
With more snow showers expected throughout the week, bookmakers have slashed the odds for a white Easter.
Sussex Police urged motorists to drive carefully as the weather deteriorated into Monday evening. It warned of heavy snow in East Sussex while the force tweeted about problems in Southwick Tunnel just outside Brighton.
A message on its Twitter page read: "Please avoid the A27 near Southwick Tunnel - vehicles stuck in tunnel!"