The largest earth tremor since fracking began in Lancashire has been recorded at Cuadrilla's shale gas site near Blackpool.
The company said there was a 2.1 ML micro seismic event detected within its operational area at 11.01pm last night (Saturday August 24), while it was not hydraulically fracturing.
It follows tremors of 1.05 at Preston New Road near Blackpool on Friday night and another measuring 0.53 on Saturday morning.
Anti-fracking protesters and residents say the tremor was felt over a wide area and have spoken of houses shaking and books falling off shelves.
The news is bound to lead to greater calls for fracking in the UK to be halted.
It comes after fracking was halted earlier this week after the then largest tremor so far - 1.55 - was detected.
Any tremor measuring 0.5 or above means fracking must be temporarily stopped.
Cuadrilla has asked for the safety limits to be increased so it can operate more effectively.
A spokesman for Cuadrilla said: “We can confirm that a micro seismic event measuring 2.1ML (local magnitude) on the Richter scale occurred at Preston New Road.
"This lasted for around one second and resulted in ground motion less than 1.5 mm/s. Hydraulic fracturing was not taking place at the time.
"All the relevant regulators have been informed and we have verified that the well integrity is unaffected"
The spokesman added: “Minor ground movements of this level are to be expected.
"Whilst this event has been felt by people on our site and some local households, it is well below anything that can cause harm or damage to anyone or their property.”
A spokesman for Frack Free Lancashire said: “We are livid that we are once again, being put in harm’s way for an experiment that we didn’t ask to be part of."
In 2011 Cuadrilla caused a 2.3 magnitude earth tremor at Preese Hall, which was felt locally. .