The heaviest snowfall to hit the UK in 18 years has brought travel chaos to large parts of the country. Thousands were left stranded as air, rail and road services suffered severe delays and thousands of schools across the country were closed. Conditions looked set to worsen overnight with forecasters predicting temperatures will plummet to -6C as the snow moves to the North. An army of snow ploughs and gritters is working around the clock to clear roads, but motorists, who were warned only to make essential journeys, were caught in tailbacks of more than 50 miles and queues of up to two-and-a-half hours. Earlier, major airports closed runways, all bus services were cancelled in London during the morning rush hour and dozens of trains - including all Southeastern services in and out of the capital - were suspended. London Mayor Boris Johnson defended the decision, saying that "unleashing a 12-tonne bus on to heavily packed snow or ice" would risk "turning it into a lethal weapon".