* Temperatures drop to minus 12 Celsius in rural areas
* 'Beast' brings Siberian weather to UK and Ireland
* Coldest snap for this time of year since 1991
* Ireland braces for heaviest snow since 1982
* Severe transport delays in London (Adds Irish and Scottish warnings)
By Guy Faulconbridge and Padraic Halpin
LONDON/DUBLIN, Feb 28 (Reuters) - Much of Britain and Ireland was blanketed in snow on Wednesday as freezing Siberian weather dubbed "the Beast from the East" disrupted the travel plans of thousands.
Hundreds of schools were closed and there were delays on roads, railways and at airports on one of the coldest days Britain has experienced at this time of the year for almost three decades.
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.
"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. "It is all tied in with the Beast from the East."
Ireland, bracing for the biggest snowfall since 1982, urged people to stay indoors from 1600 GMT on Thursday until 1200 on Friday in the southern province of Munster and eastern province of Leinster when the most severe weather is expected.
Scottish police advised motorists to avoid non-essential travel.
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.
Across London, there were severe delays on public transport. Europe's biggest airport, Heathrow, said its runways were fully operational, although its website showed that dozens of flights had been delayed or cancelled due to disruption at other airports.
Britain's weather service issued a severe weather warning for snow in parts of Scotland where schools were closed and council offices shut.
Ireland's weather service issued a status red warning - the highest level of alert - for five countries in the east, including the capital Dublin after heavy overnight snowfall led to accumulations of 5 to 10 cm.
Total snowfall may reach 25 cm (10 inches) by Thursday, it added, prompting the closure of many schools and universities. Businesses advised workers to stay at home with some roads inaccessible and public transport operating a limited service.
The chief executive of Irish insurer FBD said on Tuesday she expected the snowfall would be an event for the industry, leading to frozen pipes and possible accidents, though not as severe as Tropical Storm Ophelia which battered Ireland in October. (Writing by Guy Faulconbridge and Padraic Halpin; editing by John Stonestreet, William Maclean)