Fracking: Why the controversial gas extraction process is causing widespread concerns

Opposition to fracking has intensified in recent years over fears the controversial gas extraction process could poison drinking water and cause major environmental damage.

Cuadrilla Resources suspended test-drilling in June last year after its operation caused two small earthquakes, of 2.3 and 1.5 magnitude, which hit Lancashire's Fylde coast.

Fracking - shorthand for hydraulic fracturing - involves the blasting of water, chemicals and sand at high velocity into a shaft to crack rock and release gas.

Energy companies have said the process is safe, but environment groups warn it could pose human health risks.

Horror stories have emerged from the US, where fracking is common, including reports of tap water igniting when a match is lit and claims of contaminated water making people ill.

Such concerns have led to bans or moratoria on fracking in some places.

France has banned fracking from shale rock, while New York state introduced a moratorium.

Environmental groups in the UK have been calling for a suspension at least until environmental and safety impacts have been addressed.

The industry itself vigorously denies that shale gas is unsafe and blames pollution incidents as examples of bad practice, rather than an inherently risky technique.

There are fears shale gas may present a problem for global warming, because it is an abundant fossil fuel that could be a cheaper substitute than many renewable energy sources.

However, carbon and noxious gas emissions from natural gas are much lower than from oil and coal.

In Australia, ground water and soil contamination fears have led farmers and green groups to form an unlikely alliance against fracking.

Concerns have also been raised that fracking could pose a threat to Bath's world-famous hot springs.

Bath and North East Somerset Council has said it is worried the technique could be a major threat to the springs should resource companies get planning permission to test-drill for gas.

PA