Heavy snowfall blanketed parts of Britain on Monday, disrupting airports, train networks and roads, and three young boys died after falling into an ice-covered lake as the country recorded its coldest night of the year so far. Parts of London’s underground network were suspended or faced delays, while motorways were gridlocked due to snow.

London’s Gatwick and Stansted airports warned flight schedules could be disrupted. The Met Office issued a yellow weather warning for snow and ice in London and southeast England, with ice and fog warnings in other parts of England and the whole of Northern Ireland. Temperatures dropped below —15° Celsius in northern Scotland, with the Met Office confirming it was the coldest night of the year. Further warnings in Scotland and northeast England for the potential disruption caused by weather will start at midnight on Tuesday.

Britain’s National Grid issued a notification to prepare two winter contingency coal plants, but later cancelled the standby notices as higher nuclear output and wind speeds reduced the chance of energy shortages. The snow caused problems for commuters and holidaymakers at the start of a fortnight when rail workers and border officials plan industrial action.

Several train operators advised customers not to travel, while emergency services said it was important people take care in potentially hazardous conditions. Three boys died and a fourth — aged six — was in critical condition after they fell into an ice-covered lake in Solihull, central England, on Sunday afternoon, as emergency services continued to search the lake to check if anyone else had fallen in. “The boy’s deaths are a tragedy beyond words,” Richard Stanton from the local fire and rescue service told a news conference. “Yesterday’s incident is a stark reminder to us all of the dangers of open water, especially during the winter months.”

Emergency services were called just after 2.30pm (BST) on Sunday and local media reported members of the public had entered the water to try to help the boys. Richard Harris of West Midlands Police said that searches would continue for as long as it took until they were certain there was no one else in the lake after conflicting reports that other children might have fallen in. He said one officer was treated for mild hypothermia after going in to try to save the children, but had been released from the hospital and was fine.

“One of my officers was trying to punch through the ice to actually rescue the children,” Harris said. “The officers went straight in, without regard for their own safety. They entered the water in just normal trousers and shoes.”

He declined to comment on whether the boys were related. A spokesperson for Prime Minister Rishi Sunak said it was “devastating news for the family and friends” of the boys. “This is a tragic incident,” he said. “The Prime Minister’s thoughts are with them and obviously he passes his thanks to the emergency services who are continuing to work and provide support.”