Two earthquake tremors in north-west England earlier this year were probably caused by controversial operations to extract gas nearby, a report by the company responsible has concluded.

The two tremors - magnitude 2.3 and 1.5 - which were felt by people just outside Blackpool, but did not cause any known damage, were reported in April and May. Since the second event, Cuadrilla Resources has stopped "fracking" operations – where water and chemicals are injected into rocks at high pressure to extract gas from the cracks.

The news came as protesters against the controversial new energy source halted work at a Cuadrilla gas exploration rig at Banks, near Southport, Merseyside. Four members of the environmental campaign group Frack Off unfurled banners after climbing climbing the rig at around 5.30am on Wednesday. Others remained on the ground.

In a separate protest in London, around 50 anti-fracking activists gathered from around 3pm outside the Copthorne Tara hotel in Kensington in an attempt to disrupt an industry conference organised by by SMI international. Two dozen police kept demonstrators away from the conference on the hotel's first floor which was attended by delegates paying £1,500 per head.

The report, by a team of European seismic experts not usually employed by the company, concluded it was "highly probable" that the two main tremors and a series of aftershocks were caused by Cuadrilla's operations at the Preese Hall-1 Well in Lancashire.

It said, however, that the cause was an "extremely rare" combination of factors including a pre-existing fault in the rocks, and that it was "unlikely" to occur at other sites in the Bowland Basin, where Cuadrilla is hoping to exploit an estimated 200 trillion cubic feet of shale gas.

"If these factors were to combine again in the future, local geology limits seismic events to around magnitude three as a "worst-case" scenario," added a company statement.

The report, which also recommends an early warning system so the company can monitor work more carefully or even stop operations if seismic activity is felt in the future, will be sent to the Department for Energy and Climate Change and the British Geological Survey for approval before the company can resume operations at Preece Hall.

The study was criticised, however, by opponents of fracking, who called for a full investigation by the government before Cuadrilla or other companies are allowed to continue operations in the UK.

"An early seismic detection system won't be enough to make local people feel safe – there should be no more fracking in Britain until the health and environmental impacts are fully understood," said Tony Bosworth, senior climate campaigner for the group Friends of the Earth.

Nick Molho, head of energy policy at WWF-UK, said: "We're extremely concerned by the way in which shale gas is being painted as a 'wonder gas' which will slash energy bills in Britain and help tackle climate change. Shale gas is still a fossil fuel, and a new dash for gas could see global temperatures skyrocket. 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."

Prof Andrew C Aplin at Newcastle University said safety and pollution risks could be "handled" by good regulation, but said: "The social issue of whether the public want onshore drilling is a different matter, requiring public discussion. We should all realise that our lifestyles demand energy and that there are no easy answers to where that energy will come from."

Opinion is split over the huge and growing gas fracking industry in north America. Opponents say the industry contaminates groundwater and adds to greenhouse gas pollution from burning fossil fuels, leading to the practice being banned in some places, including France. Supporters say fracking is safe, and contributes valuable energy at a time when conventional gas and oil supplies are supposed to be running out and prices soaring.

A report into fracking by MPs, published in May, said that shale gas in the UK could improve the country's energy "self reliance" and reduce use of highly polluting coal-fired power. But said it was "unlikely" that the industry could improve energy security or lower prices, and warned that it could damage government efforts to boost renewable energy generation. "On balance", the committee recommended fracking should be allowed to go ahead so long as it was well-regulated and closely monitored.

Cuadrilla announced in September that it had "gas in-place" in its licence area in Lancashire of 200 trillion cubic feet - more than the entire UK proven gas reserves previously, and many times the British Geological Survey's previous estimate of the entire UK shale gas potential. A report commissioned by the company also estimated that the operations would create 5,600 jobs, about one-third of them in Lancashire.

Cuadrilla said groundwater in Lancashire was protected by layers of rock between the aquifers which are up to 300m deep and their operations which occur as far below as 3km underground.

A Department of Energy and Climate Change spokesman said: "Cuadrilla's geomechanical study was given to the Department of Energy and Climate Change today. The implications of this report will be reviewed very carefully - in consultation with the British Geological Survey, independent experts, and the other key regulators, [the Health and Safety Executive] and the Environment Agency - before any decision on the resumption of these hydraulic fracture operations is made."