It is "highly probable" that shale gas test drilling triggered earth tremors in Lancashire, a study has found.
But the report, commissioned by energy firm Cuadrilla, also said the quakes were due to an "unusual combination of geology at the well site".
It said conditions which caused the minor earthquakes were "unlikely to occur again".
Protesters against fracking, a gas extraction method, said the report "did not inspire confidence".
Six protesters from campaign group Frack Off climbed a drilling rig at one of Cuadrilla's test drilling sites in Hesketh Bank, near Southport, ahead of the report.
They oppose fracking, a controversial extraction method which blasts water into rock to release the shale gas, because they fear it is not safe.
A spokesman for Lancashire Police said the force was "liaising with the site owners and the protesters to bring about a peaceful resolution".
Cuadrilla suspended its shale gas test drilling in June, over fears of links to the earthquakes.
One tremor of magnitude 2.3 hit Lancashire's Fylde coast on 1 April, followed by a second of magnitude 1.4 on 27 May.
A study by The British Geological Survey placed the epicentre for each quake as being 500m away from the Preese Hall-1 well, at Weeton, near Blackpool.
The Geo-mechanical Study Of Bowland Shale Seismicity report said the combination of geological factors that caused the quakes was rare, and would be unlikely to occur together again at future well sites.
It said: "If these factors were to combine again in the future local geology limits seismic events to around magnitude 3 on the Richter scale as a worst-case scenario."
However, it said that "even the maximum seismic event is not expected to present a risk".
Cuadrilla said the report would also be subject to peer review.
A spokesman for Frack Off said: "This report does not inspire confidence, they should have done their research before drilling began."
He added: "Can we believe anything else the industry says when it talks about the safety of fracking?"
Frack Off is intending to holding a flash mob-style protest at the Shale Gas Environmental Summit in London later.
Protesters have called for an end to fracking. There have been concerns that potentially carcinogenic chemicals could escape during the process and find their way into drinking water sources.
Nick Molho, head of energy policy at World Wildlife Fund UK, reiterated a call for a moratorium on fracking in the UK.
"These findings are worrying, and are likely to add to the very real concerns that people have about fracking and shale gas," he said.
The industry denies that shale gas is unsafe and a government committee has recommended that fracking should be allowed to go ahead.
The Department of Energy and Climate Change (DECC) said: "The implications of this report will be reviewed very carefully - in consultation with the British Geological Survey, independent experts, and the other key regulators, HSE and the Environment Agency - before any decision on the resumption of these hydraulic fracture operations is made."
The process involves water and chemicals being pumped underground at high pressure to shatter the rock formations and release the gas.