Campaigners have warned that the fracking moratorium announced by the UK government does not apply to acid fracking, a process that involves injecting acid into the earth to dissolve and fracture rock.

More than 500 academics, politicians and campaigners have signed an open letter initiated by Brockham Oil Watch calling on the government to ban the practice over fears companies may use it to get around the moratorium.

Lawyers and academics said regulation of the use of acid in oil and gas wells is vague and could be exploited.

Jonathan Bartley, the co-leader of the Green party, is one of the letter’s signatories. “It isn’t acceptable just to have half a moratorium. The definition [of fracking] needs to be expanded, regulations need to apply right across all forms of unconventional drilling, and local communities need to know what’s going on beneath their feet.”

No permits have yet been granted for acid stimulation in England, although oil and gas companies, along with water companies, commonly use acid wash to clean wells.

Campaigners fear the lack of monitoring of acid use could lead to “fracking by stealth” through matrix acidising, where liquid is injected into the earth at low pressure to dissolve rock, or acid fracturing or fracking, where the acid is used at high pressure to fracture the rock.

Residents in Wressle, north Lincolnshire are waiting on the outcome of a public inquiry which took place in November last year to decide whether onshore oil and gas company Egdon can continue drilling at its site in the area. The company’s plans involve using an “proppant squeeze” that would dissolve rock around the well and can create small fractures.

The newly elected Conservative MP for Brigg and Goole, Andrew Percy has previously described fracking as “industrialisation of the countryside” and said affected communities should be given more power over the issue – although he had previously voted against tighter fracking regulation.

Stuart Haszeldine, professor of carbon capture and storage at the University of Edinburgh, said the UK government’s definition of fracking is part of the problem.

“Most countries define fracking as the intention to fracture the rock, but the UK takes the use of quite a large volume of fluid as the definition of fracking,” he said. “It’s quite possible to use a small volume of fluid to frack the rock, including acid, without actually naming it as fracking and without going through the formal permissions for fracking.”

His analysis shows that of 4,500 oil wells fracked in the US from 2000-10, 89% would not be defined as fracking in the UK, along with 43% of US gas wells fracked over the same period.

“This is a clear opportunity for gaming the system so I argue that acid treatments onshore should stop, until a coherent set of regulations is produced and with clarity about who polices those regulations.”

Campaigners claim acid stimulation brings with it many of the same risks as fracking, such as water, soil and air contamination, and earth tremors.

The Weald in Surrey and Sussex is a prime target for onshore oil and gas extraction. A local resident has launched a campaign to fund a judicial review into the council’s decision to grant 20 more years of oil production at the Horse Hill site near Horley.

Brenda Pollack, the south-east regional campaigner for Friends of the Earth, said residents near areas at risk of potential acid fracking should remain vigilant. She said: “It could be worse for those communities now because with this moratorium, companies are going to be turning to those sites where they can get their feet in the door, or their drills in the ground.”

However, industry representatives say the use of acid is standard practice and should not cause concern. Ken Cronin, the chief executive of UK Onshore Oil and Gas, said: “Acidisation, both in the water industry and our own, is a regulated practice that allows us to effectively ‘clean’ or improve recovery from a well. It dissolves fine particles and scale, allowing a better flow of what we’re trying to extract.

“As with household kettles, which need to be treated for limescale build-up, the use of this process merely increases the efficiency of our practices. An environmental permit is only issued for this technique if the Environment Agency are satisfied that the proposed activities meet the requirements of all the relevant legislation.”