A NATIONAL DAY of mourning has been declared in Belgium to honour the 22 children and six adults who lost their lives in a bus crash in Switzerland as they returned from a school skiing trip this week.

Candles lined St Jospeph’s church in Lommel, the town where from which 15 of the child victims and two of the adult victims came, euronews reports.

The Netherlands has also ordered flags on public buildings to be flown at half-mast today, with seven Dutch children among the dead.

The Belgian school group was returning from a skiing holiday in Switzerland when their bus crashed head-on into a concrete wall in a motorway tunnel at about 9pm on Tuesday.

Police said the bus was not speeding and everyone aboard had been wearing seat belts when it crashed inside the 2.5-kilometre tunnel, a gateway to the Val d’Anniviers tourist region. No other vehicles were involved.

Investigators were still trying to determine how a modern bus, a rested driver and a seemingly safe tunnel could produce one of the deadliest highway crashes in Swiss history.

One theory being reported across Swiss media is that survivors of the crash claimed the driver had reached to change a DVD on the onboard entertainment system shortly before the crash.

Swiss police have dismissed the suggestion as speculative.