Temperatures fell to minus 12 Celsius (10 degrees Farenheit) in some rural areas while Britain's weather service warned of up to 40 cm of snow in higher areas of Scotland. Ireland could see its worst snow since 1982.
"It is the coldest air we have seen over the UK at this time of the year since around 1991," Becky Mitchell, a meteorologist at Britain's Met Office, said by telephone. "It is all tied in with the Beast from the East."
The cold spell has been caused by a jump in temperatures high over the Arctic, known by meteorologists as sudden stratospheric warming, which has weakened the jet stream that brings warm air in from the Atlantic to Ireland and Britain.
"The Jet Stream weakens and so we tend to get a strong easterly flow of air and that air is really originating right from over in Siberia - so very dry, cold continental air," Mitchell said.
The unusual weather could continue as Storm Emma, packing more snow and ice, approaches western England from Portugal and France.