Concerns have been raised in many countries about the safety of fracking, a technique in which high-pressure fluids are pumped into shale to fracture the rock and force out natural gas.

And technological improvements have made the huge reserves of gas in America, from shale beds and the like, a commercial prospect.

Producers have boosted supplies and recoverable reserves in the United States by combining horizontal drilling with hydraulic fracturing, also called fracking, to produce gas from shale formations.