Shale gas is natural gas locked in rock formations. In the past decade it has been found in great abundance around the world and is now considered a top source of future energy.

Castro, 85, wrote in one of his occasional columns published in Cuban state media that "numerous dangers threaten us, but two of them - nuclear war and climate change - are decisive and both are ever further from approaching a solution".

He said he had heard only recently about the shale gas phenomenon, and when he asked several acquaintances both in and outside Cuba about the topic "none of them had heard a word about it".

Shale gas production is criticised in some quarters because it requires extensive "fracking", which uses water, sand and chemicals to fracture the rock in which the gas is trapped to allow it to flow from the well. Fracking, critics say, contaminates groundwater.

Castro sided with them, quoting reports on the negative effects of fracking and research that said shale gas emitted more greenhouse gases than the gas produced from conventional wells.

"It is sufficient to point out that among the numerous chemical substances injected with the water to extract this gas is found benzene and toluene, which are terribly carcinogenic," he wrote.

The information on shale gas was something "no political cadre or sensible person could ignore".

Castro said he was so intent on getting the word out that he had "let the festive days of the old and new year pass by" working on his column.