An earth tremor has halted fracking in an area previously linked to earthquakes.
Drilling for shale gas at Little Plumpton in Lancashire was given the green light on 15 October, despite strong opposition.
Energy firm Cuadrilla said the seismic event on Tuesday, measuring 0.4, was extremely low level and could not be felt on the surface.
It halted the process as a precaution on Tuesday, but it has since resumed.
Seven years ago a 2.3 magnitude tremor on the Fylde coast was probably caused by shale gas test drilling, a study found.
Cuadrilla said earlier this month it would spend at least three months fracking two horizontal wells and then test to see if the gas flow was commercially viable.
Anti-fracking campaigners tried to stop the process earlier with an attempted injunction but failed in their bid.
Test drilling was suspended after two tremors, one of 2.3 magnitude and the other of 1.5 in April and May in 2011 respectively.
A government-appointed panel said there could be more tremors as a result of fracking but they would be too small to do structural damage above ground.
It recommended greater monitoring and said operators should observe a "traffic light" regime, with tremors of 0.5 or above triggering a "red light" and an immediate halt.