The Czech News Agency said mine operator OKD would not speculate about the cause of deaths but the news service said it was likely the mine rescuers inhaled poisonous gases.

“We can only say for sure there was no cave-in, explosion, ground tremor or similar event," OKD spokesman Vladislav Sobol said.

“Any other information would be mere speculations.”

Both men were 44 years old and died 1100m under the surface.

They were employees of the mining rescue station HBZS Ostrava, a daughter firm of OKD, that deals with the taking of samples, maintenance of mining equipment, work safety rules and rescue works in accidents.

It is the first underground fatality in the area this year and an investigation is underway.

OKD is the only producer of hard coal in the Czech Republic.

It has four active mines in the southern part of the Upper Silesian Coal Basin in the Ostrava-Karviná coal district.