A total of 43 workers launched the protest on Thursday in their mine in Guardo, in the northern province of Palencia, to demand their employers pay their wages for August, the spokesman for the Union Minera del Norte, Juan Jose Valverde, told AFP.

He said they were joined at the weekend by eight colleagues, and now all fear they may not be paid for September either.

Voluntarily enclosed in the coal mine, the 51 miners have warned that their protest is “indefinite.”

The mining sector in Spain is going through a difficult period, with electricity generating stations using less Spanish coal, which is too expensive compared with that of other countries.

“To confront the overproduction, we need a regulated market. And the most important for us use of the coal that we produce is guaranteed,” said Valverde.

The European Commission proposed in July that subsidies for loss-making mines should end by October 2014.

Spanish coal is used for around 25 percent of electricity consumed in the country.