Millions of people are expected to travel for Christmas today as a break in snowfall gave welcome relief after the worst winter weather for a century.

Roads and rail tracks were expected to get respite from snow but forecasters warned of sharp frosts that would leave widespread ice on the ground.

And train passengers were warned to expect a reduced service with three rail operators due to cut their timetables by a quarter.

Temperatures overnight were due to plummet to as low as minus 15C (5F) in remote parts of Scotland and were unlikely to get above freezing for most of the country today.

But while some parts could see the occasional flurry, most of the country will escape heavy snow. The news will bring some late Christmas cheer to the thousands of passengers waiting to travel at Heathrow Airport, which was forced to close earlier this week because of the weather.

BAA launched an inquiry on Thursday into its planning, execution and recovery from the extreme weather that left planes grounded and passengers bedding down in terminals. An international panel of experts from airports and airlines around the world will make recommendations to BAA.

Chief executive Colin Matthews said: "The inquiry will forensically examine what went wrong at Heathrow, and look fundamentally at our ability to prepare and respond more effectively to periods of bad weather at an airport operating at its maximum capacity."

Forecasters said London and the South East would see a relatively mild frost today but temperatures would be much colder elsewhere.

Matt Dobson, forecaster with PA's weather division MeteoGroup, said: "The cold air is not going anywhere fast. Away from the south east of England where the frost is going to be a lot slighter, there's going to be a very sharp frost. Parts of Northern Ireland could be getting close to another record-breaking temperature.

He added there was a "small chance" of snow falling very early on Christmas Day across Kent, East Anglia and Lincolnshire with the highest chance of snowfall in Scotland. Bookmaker Ladbrokes stopped bets on a white Christmas on Thursday but Mr Dobson said most parts were expected to see a dry, sunny and cold day.