Local authorities are running out of salt and grit as they struggle to cope with further deluges of snow that are closing roads, schools and businesses. Many paths and roads have still not been cleared from this week's earlier flurries, yet more snow is falling across the UK bringing services to a halt once more. Local authorities are struggling to cope with the task ahead as Britain's biggest salt supplier, Cheshire-based Salt Union, said staff are working round the clock but still cannot meet demand. The Highways Agency - which is responsible for England's motorways and A roads - said it is confident it has enough salt in its depots to keep routes ice-free. In the Commons, shadow leader Alan Duncan called for a debate on the "lack of preparedness in this country for snow and ice". He poked fun at Commons Leader Harriet Harman, saying: "Perhaps you can dismiss the scurrilous rumours circulating this week that you spent most of Monday building a snowperson. "We are now learning that some councils have been forced to scale back and even halt their gritting and salting programmes due to national shortages." Thousands of schools across the country remain closed - at least 300 in Scotland alone - as pupils enjoy another day off. Health campaigners have urged caution after a group of teenagers in Knowle were spotted hurtling down a hill in a bath. It came just two days after 16-year-old Francesca Anobile died sledging on an upturned car roof. Peter Cornall, head of leisure safety at the Royal Society for the Prevention of Accidents (Rospa), said: "Obviously at an organisation like Rospa we're keen to reduce the risk - but we still want people to have fun. "If you're improvising, like the kids in the bath, you need to think about 'what if?' You need to weigh up the steepness of the slope, your ability to be able to stop yourself, whether you'll hurt others if you slide into them, what equipment you're wearing." Meanwhile, Luton Airport cancelled 13 flights as clearing work took place and planes were also unable to land and take off from Belfast City airport, Robin Hood International Airport, near Doncaster, and Aberdeen. The runway at East Midlands Airport near Derby was briefly closed, while Birmingham Airport remained open with temporary runway closures. The railways are also enduring a fourth successive day of disruption, with services in Wales, the Midlands, the South West and Yorkshire affected. The Highways Agency repeated its warnings for drivers to take extra care, particularly on the M4 and M5 in Wiltshire, where driving conditions were especially dangerous.