The tremor struck mid Wales on Monday night, prompting dozens of people to telephone the emergency services.

Glenn Ford, a seismologist at the British Geological Survey in Edinburgh, said the tremor was "significant" for the UK, although he admitted it was small on a world scale.

He said: "It would be regarded as minimal in other parts of the world.

"There are at least 50,000 a year of that size. But in UK terms it's very significant. We perhaps only have two or three in the UK each year of that size."

'No reports of injuries'

Mr Ford, who helps monitor earthquakes across the world, said the earthquake measured 3.5 on the Richter scale.

The earthquake's epicentre was reported as Sennybridge, west of Brecon, and had a radius of between 20 and 35 miles.

A spokesman for Dyfed-Powys Police said the tremor appeared to have been strongest in Brecon.

BBC's Sue Cass reports on the Brecon tremor

He said so far there had been no reports of injuries or damage to properties in the area.

Seismologists are assessing data to work out the exact size of the earthquake.

Police said telephone calls had been received from worried residents as far apart as Hay-on-Wye, on the England/Wales border, Aberdare, in south Wales, and Rhayader, in mid Wales.

'I thought it was a big gun'

Alex Jones, 31, from Felinfach, near Brecon, said he was on the telephone when he heard at rumble lasting a few seconds just after 8pm.

He said: "We live quite near an Army firing range so I thought it was a big gun at first or a low-flying aircraft. But when I spoke to my father who lives nearby he said he thought it was an earthquake. His whole house shook."

A Brecon woman said she went outside after the tremor to find all her neighbours there wondering what had happened.

"It went on for quite a few seconds. No-one seemed to know what it was. It was quite worrying," she said.

The biggest on-shore earthquake to hit the UK this century, which measured 5.4 on the Richter scale, hit north Wales in 1984, said Mr Ford.