Former Tour de France winner Sir Bradley Wiggins is among the race favourites along with Nairo Quintana, second behind Britain's Chris Froome in this year's Tour.
This year's race starts on the Scottish Borders and will move through the Lake District, Snowdonia and Dartmoor before concluding with a showpiece finish in central London.
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Another cold, frosty and foggy night for many. Northern and western areas will see thickening cloud, patchy rain and milder temperatures.
Temperatures have already plunged to -12C in some parts of the UK this winter - but Thursday could be the coldest day yet.