Shale gas producer Cuadrilla Resources said its hydraulic fracturing process likely triggered two small earthquakes in northern England this year.

Cuadrilla issued a report Wednesday confirming it was "highly probable" that a 2.3-magnitude tremor and a 1.5-magnitude quake felt in Lancashire, England, in April and May were due to "fracking" at the company's shale gas wells there.

Under the process, rocks are fractured thousands of feet underground using high-pressure water, sand and chemicals to free natural gas trapped in the formations.

The procedure has come under heavy criticism from environmentalists, who say it has the potential to pollute groundwater as well as cause earthquakes.

France in June banned shale gas explorations due to the concerns.

"The seismic events were due to an unusual combination of geology at the well site coupled with the pressure exerted by water injection as part of operations," Cuadrilla Resources said.

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

The report also blamed fracking for 48 weaker seismic events in the Bowland Basin area northeast of Blackpool, England.

Cuadrilla Resources Chief Executive Mark Miller said the company "unequivocally accept the findings of this independent report and are pleased that the report concludes that there is no threat to people or property in the local area from our operations."

Miller offered to install an early detection system to "provide additional confidence and security to the local community."

Cuadrilla has touted its Bowland Basin leases as holding some 200 trillion cubic feet of shale gas -- enough to satisfy Britain's gas needs for 64 years -- although the amount it actually can recover would likely be far lower.

Its fracking activities were halted after a preliminary study by the British Geological Survey indicated they were responsible for the seismic events.

Charles Henry, minister of state for Britain's Department of Energy and Climate Change, said his agency "will look at Cuadrilla's report carefully with the assistance of our independent experts and regulators, before deciding whether hydraulic fracturing operations should resume."

Henry cautioned that although the potential for unconventional gas is worth exploring because of "the additional security of supply and economic benefits it could provide," the scale of any possible commercial production is "still unknown" due to unanswered environmental questions.

Cuadrilla's report also brought denunciations from environmentalists, who say the promotion of shale gas by energy producers and European governments is being filtered through rose-colored glasses.

Green Party Member of Parliament Caroline Lucas called on Henry to continue the moratorium on fracking.

"This study raises yet more questions about Cuadrilla's controversial operations in the north of England -- and further highlights the need for a thorough and fully independent investigation into the environmental and health impacts of fracking," she told the Financial Times.

"It just serves to confirm one of our many fears about shale gas," the environmental group WWF-UK said in statement, citing the tremors as just one the problems with shale gas drilling.

"Shale gas is still a fossil fuel and a new dash for gas could see global temperatures skyrocket," warned Nick Molho, WWF-UK's head of energy policy. "There's also no evidence that it will have a big impact on energy bills, which have in fact been driven up in recent years by a rising gas price."