Six workers died and another five were injured, one of them seriously, following a methane gas leak on Monday afternoon at the Hullera Vasco-Leonesa coal mine in Llombera de Gordón, León.
The workers who died were aged between 35 and 41 years, five being from Asturia and one from León.
Five other workers were transferred to the Complejo Hospitalario in León, one of whom is said to be in a critical condition.
The incident occurred at around 14:00 and is thought to have been caused by the escape of a pocket of poisonous gas, although an official investigation is under way to determine exactly what happened.
The Junta de Castilla y León has declared three days of official mourning as from tomorrow, Tuesday.