* Policy change comes weeks before general election

* "We cannot be certain shale gas can be extracted safely"

* Opposition Labour party derides move as election stunt (Adds Labour reaction)

By Stephen Addison and Alistair Smout

LONDON, Nov 2 (Reuters) - Britain will impose an immediate moratorium on fracking, the government announced on Saturday, saying the controversial gas extraction technique risked causing too much disruption to local communities through earth tremors.

The move could win support for Prime Minister Boris Johnson's Conservatives in constituencies in northern England where fracking had been planned, but was dismissed by the opposition Labour party as a "stunt" ahead of December's election.

The government had previously signalled its support for the shale gas industry as it seeks ways to cut Britain's reliance on imports of natural gas which is used to heat around 80% of Britain's homes.

But fracking, which involves extracting gas from rocks by breaking them up with water and chemicals at high pressure, is fiercely opposed by environmentalists who say it is at odds with Britain's commitment to reach net zero carbon emissions by 2050.

Business Secretary Andrea Leadsom said that the government could not rule out unacceptable impacts on the local communities in the future if fracking continued.

"We've always been clear that we will follow the science," she told BBC radio. "We cannot be certain that shale gas can be extracted safely, and therefore we must impose this moratorium until the science changes."

Saturday's announcement comes as Johnson gears up for an election on Dec. 12. Proposals for developing fracking in several marginal seats in northern England will now be postponed.

"The Conservatives' temporary pause of fracking is an election stunt to try and win a few votes," opposition Labour party leader Jeremy Corbyn said in a tweet.

"Boris Johnson described fracking as 'glorious news for humanity'. We cannot trust him. Labour would ban fracking. That's real change."

IMMEASURABLE TOLL

The decision follows a report on an incident at a site run by British energy company Cuadrilla near Blackpool, northern England where a 2.9-magnitude tremor shook houses in August.

An anti-fracking campaign by local people emerged as a flashpoint in a growing climate activist movement opposing new fossil fuel projects around the world. Hundreds of protesters have been arrested over the past few years for trying to disrupt Cuadrilla's operations.