UK shale gas firm Cuadrilla Resources on Wednesday said exploration work triggered small tremors at its drill site near Blackpool in northwest England earlier this year, as activists scaled a rig protesting against its gas recovery methods.

It is highly probable that the hydraulic fracturing of Cuadrilla's Preese Hall-1 well did trigger a number of minor seismic events, a report commissioned by the company said.

A spokesman later confirmed the tremors were triggered by pumping vast quantities of water at high pressure 3 kilometres underground through drill holes in a process known as hydraulic fracturing, or fracking, which is designed to prop open shale rocks and release trapped gas.

It said the region's unusual geology contributed to tremors in April and May of 2.3 and 1.5 on the Richter scale, respectively.

Britain suspended fracking following the May tremor and commissioned a report into the process but Cuadrilla has since said there was little risk of tremors caused by exploration work in future.

In a statement, Cuadrilla said the region's combination of geological factors is extremely rare and would be unlikely to occur together at future drilling sites.

Provided that assumption proved false, it capped the magnitude of any future tremors at around 3 on the Richter scale as a worst-case scenario.

Cuadrilla's water injection operations take place very far below the earth's surface which significantly reduces the likelihood of a seismic event of less than 3 on the Richter scale having any impact at all on the surface, the company said.

The Lichfield-based company has valued the amount of shale gas in place -- not the recoverable volume -- at its site near Blackpool at 200 trillion cubic feet, the top end of what is estimated can be recovered from Europe's largest reserves in Poland.

ACTIVISTS SLAM FINDINGS

Couadrilla has proposed implementing a seismic early warning system to make local people feel safer. But activist and environmental groups say the measure masks the real risks of fracking in Britain.

Responding to findings, WWF-UK has called for a moratorium on shale gas exploration until environmental risks have been properly assessed.

France banned shale drilling in July in the face of concerns about potential environmental damage due to the large amounts of water and detergents used in fracking.

These findings are worrying, and are likely to add to the very real concerns that people have about fracking and shale gas, Nick Molho, head of energy policy at WWF-UK said.

On top of earthquake risks, the U.S. experience shows shale drilling sucks funding away from renewable energy projects while at the same time polluting air and water supplies, according to Friends of the Earth.

However, legal experts say findings presented to the UK's Department of Energy and Climate Change signal that a suspension on fracking in the UK is not likely to last.

In the US, where significant shale gas development has been underway for many years...there have been very few instances of water contamination, Lynne Freeman, partner at law firm Reed Smith said.

Those instances have largely resulted from surface spills or improperly cemented well casing, and the fracking process has earned a strong safety and environmental record, she said.

PROTESTERS STORM RIG

Ahead of the fracking report's publication, nine protesters from the UK's anti-fracking group Frack-Off ran on to the company's drilling site at Hesketh Bank before dawn and scaled the rig using climbing equipment, the group said.

Frack-Off says it intends to draw attention to the harmful effects of shale drilling on local environments and bring operations to a standstill for as long as possible.

Cuadrilla said it is working with police to remove four protesters attached to a rig.

(Reporting by Oleg Vukmanovic; editing by Keiron Henderson)