Drivers and commuters have faced difficult journeys in icy conditions across parts of the UK, following another night of sub-zero temperatures.
The Met Office has extended yellow warnings for snow and ice until 11:00 GMT and the AA warned driving could be "hazardous".
Hundreds of schools are remaining closed for a second day.
A low of -13C (9F) was recorded on Monday night in Shawbury, Shropshire - the coldest night of the year so far.
The Met Office's warning covers Wales, parts of Scotland, Northern Ireland and much of England - including the Midlands, Yorkshire and Humber, London, the South East, East, South West, the North East and North West.
It said: "There will probably be icy stretches on untreated roads, pavements and cycle paths with some injuries possible from slips and falls.
"Some roads and railways are likely to be affected with some journey times taking longer."
BBC Weather's Carol Kirkwood warned it would be a cold start to the day, with many areas seeing frost, lying snow, ice and freezing fog.
AA president Edmund King said: "We expect Tuesday to be extremely busy as temperatures plummet overnight, causing even more hazardous conditions on already wet and slippery roads."
It had its busiest day of the year on Monday, with about 25,000 calls from motorists.
More than 350 schools in the West Midlands and more than 300 schools in Wales have said they are not opening.
On Monday, more than 1,000 schools were closed - about 600 of which were in Wales.
The deep freeze also left thousands without power. There were 220 homes still without supply in the West Midlands early on Tuesday.
National Rail is telling travellers to check with train operators before heading out following a day of delays.
Arriva Trains Wales, Great Western Railway, Greater Anglia and London Northwestern Railway were reporting major delays on Tuesday morning.
Eurotunnel said services were running with delays due to "extreme weather conditions", with waits of six hours for passengers departing from Calais and those leaving from Folkestone.
It recommended customers cancelled or changed their travel plans.
A low of -11.6C (11F) was recorded on Sunday night in Chillingham Barns, Northumberland, with Saturday's temperatures dipping to -12.4C (10F).
Forecasters said temperatures would slowly warm during Tuesday but freezing fog patches are expected to linger in areas.