Travellers are facing blocked roads and disrupted rail and air transit with ice covering many parts of Britain.
Budget airline easyJet called off around 70 flights in and out of Gatwick Airport, although the runway did reopen on Wednesday night.
British Airways (BA) said it had also cancelled several flights while many more were delayed due to icy conditions at both Heathrow and Gatwick.
BA is allowing passengers expecting to fly on Thursday to rebook their flight for another day.
"We understand that some customers (today) will have difficulty travelling to the airport so we are also offering the option of rebooking flights to a date within the next 14 days regardless of whether the flight has been cancelled," the airline said.
Luton, Birmingham, Stansted, Cardiff, Glasgow and Southampton Airports were among those open and working back to normal levels.
But they are advising travellers to expect disruption and to contact their airline for information.
Forecasters warned motorists of dangerous ice on untreated routes, even with more of the UK's major roads opening up.
Several police forces including Devon and Cornwall, Kent and Lothian and Borders are urging people to avoid travel unless necessary.
© Press Association 2009