Serious and imminent danger is the legal threshold in section 44 of the Employment Rights Act 1996. If you reasonably believed you or others faced this level of risk, your employer can’t penalise you for refusing to do something because of it.
In practice
A group of prison officers refused to drive onto a road closed after snowfall, believing it posed a serious and imminent dangerSerious and imminent danger is the legal threshold in section 44 of the Employment Rights Act 1996. If you reasonably believed you or others faced this level of risk, your employer can't penalise you .... The tribunal had to assess what each of them individually knew and believed at the time.
Legislation covered