What is fracking and why is it controversial?

The ban on fracking in England will be reinstated, new Prime Minister Rishi Sunak has said. It reverses a decision by his predecessor Liz Truss.

Fracking was first halted in England in 2019, amid opposition from green groups and concerns about earth tremors.

Hydraulic fracturing, or fracking, is a technique for recovering gas and oil from shale rock.

It involves drilling into the earth and directing a high-pressure mixture of water, sand and chemicals at a rock layer, to release the gas inside.

Wells can be drilled vertically or horizontally in order to release the gas.

The injection of fluid at high pressure into the rock can cause earth tremors - small movements in the earth's surface.

More than 120 tremors were recorded during drilling at a Cuadrilla site in Blackpool, including one which lasted almost 100 hours.

Seismic events of this scale are considered minor and are rarely felt by people - but are a concern to local residents.

Shale gas is also a fossil fuel, and campaigners say allowing fracking could distract energy firms and governments from investing in renewable and green sources of energy.

Fracking also uses huge amounts of water, which must be transported to the site at significant environmental cost.

Rishi Sunak was asked about fracking at his first Prime Minister's Questions, by Green Party MP Caroline Lucas.

He told the Commons he "stands by" the 2019 Conservative Party manifesto, which pledged to maintain a ban on the practice in England.

It had banned fracking earlier that year and stated that it would not be allowed unless the science changed. A scientific review into fracking by the British Geological Survey says there is still a limited understanding of the impacts of such drilling.

Mr Sunak's stance reverses a decision taken by the government in September, when Liz Truss was prime minister.

At that time it said fracking could go ahead in some places. It said it could help the UK strengthen the security of its energy supplies, amid uncertainty caused by the war in Ukraine.

The Scottish and Welsh governments continue to oppose fracking, and say they will not use their powers to grant drilling licences.

Fracking for shale gas in the UK has only taken place on a small scale and has faced several public and legal challenges.

However, exploration has identified large swathes of shale gas across the UK, particularly in northern England.

More than 100 exploration and drilling licences were awarded to firms including Third Energy, IGas, Aurora Energy Resources and Ineos.

Cuadrilla was the only company given consent to begin fracking.

It drilled two wells at a site in Lancashire but faced repeated protests from local people and campaigners.

The Oil and Gas Authority told Cuadrilla to permanently concrete and abandon the wells.

It is not clear what difference restarting fracking would make to energy bills.

The UK can only meet 48% of its gas demand from domestic supplies (this would be 54% if it did not export any gas).

A small number of Tory MPs, known as the Net Zero Scrutiny Group, claimed restarting drilling at Cuadrilla's two existing wells could be done quickly, and would provide significant supplies.

Cuadrilla claimed that "just 10%" of the gas from shale deposits in Lancashire and surrounding areas "could supply 50 years' worth of current UK gas demand".

Energy experts dispute this, pointing out that the UK's shale gas reserves are held in complex layers of rock.

Mike Bradshaw, professor of global energy at Warwick University, says estimates of how much shale gas the UK has are not the same as the amount of gas that could be produced commercially.

And BBC business editor Simon Jack recently shared a quote made by Kwasi Kwarteng when he was business secretary:

"No amount of shale gas from wells across rural England would be enough to lower European price any time soon."

It is thought that fracking has given energy security to the US and Canada for the next 100 years, and has presented an opportunity to generate electricity at half the CO2 emissions of coal.

Fracking remains banned in numerous EU countries, including Germany, France and Spain, as well as Australia.

Authorities in countries including Brazil and Argentina are split, with some banning the practice, and others allowing operations.