Cuadrilla is drilling a vertical well and horizontal bore near the village of Balcombe
Energy firm Cuadrilla has said it is scaling back work at an exploration site in West Sussex on police advice.
The company has been drilling for oil near the village of Balcombe but has not ruled out using the controversial technique of fracking to release gas.
Police believe environmental activists at the site are about to begin a campaign of civil disobedience.
About 1,000 extra campaigners are expected to join existing protesters this weekend, according to police.
Fences erectedExploration operations will be effectively at a standstill just two weeks after drilling got under way at Balcombe.
The number of workers on the site has been reduced and large reinforced fences are being erected.
Cuadrilla said in a statement: "After taking advice from Sussex Police, Cuadrilla is scaling back operations ahead of this weekend's No Dash For Gas event.
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BBC's John Moylan said Cuadrilla had put the site into lock-down mode to ensure the safety of staff, villagers and protesters
"During this time, our main concern is the safety of our staff, Balcombe's residents and the protesters following threats of direct action against the exploration site.
"We will resume full operations as soon as it is safe to do so."
Cuadrilla is drilling a 3,000ft (900m) vertical well and a 2,500ft (750m) horizontal bore but said fracking for shale gas would need fresh permission.
'Take back power'Environmental campaigners have been encamped at the site for the past three weeks, with about 40 people having been arrested in that time.
Sussex Police believe more people will arrive for a six-day camp organised by the No Dash For Gas group.
It warned it would engage in mass civil disobedience - last year members occupied at West Burton power station in Nottinghamshire.
No Dash for Gas said: "There are two stories that could emerge from Balcombe this summer.
"It could be the place that paved the way for a dirty and dangerous method of fuel extraction to tear up the country, or it could be the place where a group of ordinary people inspired the world by taking back the power.
"We are here, together with dedicated people from Balcombe, to make sure that it's the latter."
In an open letter, Balcombe Parish Council chairman Alison Stevenson called on the group not to break the law.
Fossil fuelsSupt Lawrence Hobbs, of Sussex Police, said: "We are acutely aware of the impact that this is having on the residents of Balcombe and back their call to protesters not to engage in any criminal activity in the pursuit of their aims.
"We will continue to facilitate peaceful protest but newcomers to the site should be aware that if they commit criminal offences then we will collect the evidence and they will be arrested."
Cuadrilla has said it is "unlikely" to use the site for the production of fossil fuels.
Both Friends of the Earth and Greenpeace have spoken out against the company's activities.
Prime Minister David Cameron has urged the UK to embrace fracking - short for "hydraulic fracturing" - as an important source of energy production.
But opponents of the technique claim it can cause water contamination and environmental damage, and trigger small earth tremors.
Source: BBC News - Business http://www.bbc.co.uk/news/uk-england-sussex-23721713#sa-ns_mchannel=rss&ns_source=PublicRSS20-sa
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