LONDON — A controversial method of extracting gas from the ground known as fracking was the "highly probable" trigger of earth tremors along England's coastline this year, according to findings published Wednesday.

British energy firm Cuadrilla Resources said a study of its drilling along Lancashire's Fylde coast, northwest England, concluded "it is highly probable" that the fracking "did trigger a number of minor seismic events."

A tremor of magnitude 2.3 on the Richter scale was recorded in the area in April followed by a second of magnitude 1.5 in May.

"The seismic events were due to an unusual combination of geology at the (Preese Hall-1) well site coupled with the pressure exerted by water injection as part of operations," Cuadrilla said, citing a report it had commissioned.

"This combination of geological factors was extremely rare and would be unlikely to occur together again at future well sites," it added.

Cuadrilla chief executive Mark Miller said the company "unequivocally accepts" the findings of the report, carried out by a team of independent seismic experts.

Fracking is the drilling of underground shale rock formations by injecting chemicals and water to release trapped natural gas. Opponents of the process say it causes water pollution but energy groups say it drives down gas prices.

Last month the US government unveiled plans to set national standards for wastewater discharges from fracking.