People have reported their houses swaying after a magnitude 4.1 earthquake three miles beneath the Bristol Channel.
The British Geological Survey (BGS) said the tremor, which happened at 1.21pm, was being analysed.
No aftershocks had been recorded but were not unusual following an earthquake, a spokesman said.
The tremor is thought to have lasted several seconds.
Residents reported their houses rocking and others described the earthquake as like a "heavy truck going up the road".
It was felt in areas including Dartmoor, Bristol, Taunton, Swansea, Llanelli, Barnstaple and Gloucester, according to the BGS.
Robin Beer tweeted: "Anybody else feel the earth tremor just now?!"
Glenn Watts added: "Small tremor felt in Exeter. Lasted five seconds. Armchairs wobbled, as did the TV on its stand."
People in South Wales also reported feeling the ground shake.
One tweeter wrote: "Earthquake tremor in Wales today. We felt it in the building in Swansea."
Brian Baptie, from the BGS, told Sky News: "We get an earthquake of this size maybe roughly once every three years somewhere in the UK.
"It appears to have been felt fairly widely in Devon and South Wales.
"It is possible for earthquakes of this size to cause some minor, superficial damage, but we have no reports of that nature at the moment."
The UK experiences up to 300 earthquakes each year but most are so small people do not notice them. Only around 20 are actually felt.
A spokesman for the BGS said: "It's a very typical event for the UK."
The European-Mediterranean Seismological Centre (EMSC) said the tremor was a magnitude 4.5 earthquake and was centred around Exmoor National Park.
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