LONDON (Reuters) - Shale gas is safe and Britain needs to revisit the limits on seismic activity allowed at fracking sites, business minister Jacob Rees-Mogg said on Thursday.

"We need to revisit the seismic limits to ensure that shale gas extraction can be done in an effective and efficient way," Rees-Mogg told parliament after the government formally lifted a moratorium on fracking for shale gas.

"We know that shale gas is safe. It is safe in the United States."

The largest tremor caused by fracking in Britain took place at a site in Blackpool, northern England, in 2011, registering a magnitude of 2.3 which residents said woke them in the night.

After this, the government introduced a traffic-light system which suspended work if seismic activity of 0.5 or above on the Richter scale was detected.

"Seismic activity of 2.5 and below on the Richter scale takes place millions of times a year across the world," Rees-Mogg said.

"Our standards for ground level movements for construction work are double those that have ever been achieved by any shale gas exploration in this country."

(Reporting by Muvija M, Writing by Kylie MacLellan; editing by William James)