Fracking has been halted at Cuadrilla’s gas exploration site in Lancashire after tremors were detected underground.
The British Geological Survey recorded a 0.8 magnitude earthquake in the region of the site at about 11.30am on Friday.
It is the largest tremor at the Preston New Road, Little Plumpton, site since fracking began this month.
But the exploration company said “micro seismic” events like this do not cause any damage and cannot be felt at ground level.
“Events such as these result in tiny movements that are way below anything that would be felt at surface, much less cause any harm or damage,” a spokesman said.
The quake, which took place 2km underground, was classed as a red event under the UK’s fracking regulation traffic-light scheme, meaning it was more than 0.5M.
Cuadrilla was hydraulically fracturing the shale rock adjacent to a well when the seismic activity was recorded, the spokesman said.
“All the relevant regulators were informed without delay and we have verified that the well integrity is intact,” he said.
The controversial fracking work began 11 days ago after an environmental campaigner failed in a High Court bid to block it.
The exploration is the first fracking in the UK for seven years, after work by Cuadrilla was halted in 2011 following two tremors near Blackpool.
Friends of the Earth campaigner Rose Dickinson said fracking poses risks to the environment and the latest tremor was “deeply concerning”.
“Fracking only started 11 days ago. In that time there have been 17 earthquakes, including one today that has reached a red warning level,” she said.
“This is deeply concerning for those living nearby and why the industry must be closely monitored.
“When is the Government going to realise that fracking is the wrong choice for Lancashire, the UK and our global climate?”
Friends of the Earth has helped to organise a petition, signed by some 300,000 people, to stop the fracking through the planning system.
A spokesman for BGS said Friday’s quake was recorded “right on top” of the Caudrilla site and was the largest of 17 tremors there this month.
“It’s been caused by the hydraulic fracking,” he said.
Cuadrilla chief executive Francis Egan said fracking will continue on Monday and nobody should be worried about the tremors.
“There is no chance this is going to cause damage or harm,” he told BBC News.
“Nobody should be concerned in any way shape or form.”
Mr Egan said the red event threshold had been set at an “extremely conservative level”.
“If you drive a bus down the road you’ll get a ground motion that is greater than 0.5 (magnitude),” he said.
“When you create tiny fractures in the rock we can detect that.
“But the reason we’re doing that is because the country needs gas.”