The UK government has been accused of including a large loophole in its legal definition of fracking which could enable companies to bypass safety regulations, according to a leading geologist.

In rules that came into force on 6 April, fracking is defined by the amount of high-pressure fluid used to fracture shale rocks and release gas or oil. However, the only well fracked in the UK so far, which caused small earthquakes near Blackpool in 2011, would not qualify as fracking under the definition.

Furthermore, according to Prof Stuart Haszeldine at the University of Edinburgh, analysis of more than 17,000 gas wells fracked in the US from 2000-10 shows 43% would not be defined as fracking under UK rules. More than 4,500 US wells were fracked to release oil in that time but 89% would not be covered by the UK definition.

The safety regulations in the new rules, such as independent inspection of the integrity of the well and sealing it after use, only apply if the drilling activity is defined as fracking.

“If a UK well is not officially ‘fracked’, then there is very large loophole, where a developer may not be enforced to take all these additional safety precautions,” said Haszeldine, who has just returned from a visit to the heartland of US fracking in Pennsylvania. “Once a loophole exists, then companies will exploit that loophole.

“We have these detailed rules and regulations, a lot of which are based on mistakes made in the US, so I really don’t understand why you would enable companies to bypass them,” he told the Guardian.

A spokesman for the Department of Energy and Climate Change said: “Our definition of hydraulic fracturing is consistent with the approach taken by the European commission and we are confident that the right protections are in place to ensure that it will be safe.”

Fracking in the UK is defined in the new law as “the injection of more than 1,000 cubic metres of fluid at each stage (or expected stage), or the injection of more than 10,000 cubic metres of fluid in total”. The government argues that using smaller volumes than this would not produce much gas or oil and notes that the two applications made so far, by Cuadrilla and Third Energy, are defined as fracking.

“Getting much oil or gas out isn’t really what we might be worried about,” said Haszeldine. “I’m more interested in the safety precautions appropriate to injecting high-pressure water with additives, and the precautions against gas leakage during evaluation and after abandonment.” He said it is clear from the US that boreholes can be drilled and securely fracked if properly regulated.

The shadow energy minister, Alan Whitehead, said: “It’s no wonder there is significant public concern over fracking when the government are so determined to avoid imposing the strong safeguards we need to reduce the environmental risks. No fracking should go ahead until tougher rules are in place to protect drinking water and risks to our precious countryside.”

However, a spokesman for industry group UK Onshore Oil and Gas said: “It is highly unlikely that that high-volume hydraulic fracturing will involve less than 1,000m3 per stage. However in any event the same regulatory environment applies for well integrity whatever volume is used. In addition operators will have to apply for environmental permits which will also include the chemicals being used.”

The new rules, part of the Infrastructure Act, have previously caused controversy. The government was criticised for allowing fracking under national parks and sites of special scientific interest, despite the energy secretary, Amber Rudd, having earlier announced “an outright ban on fracking in national parks”.

The government said in July 2015 that the fracking industry would be developed with “world-class environmental protections”. Haszeldine said: “If so, then the law they are working to, needs to be clear and enforced.” He said there were examples from the US of water being contaminated with dangerous chemicals if fracking is not properly regulated.

“If the government were as serious as they claim to be about regulating fracking, they would have chosen a definition of this technique that reflects what actually goes on at many drilling sites,” said Hannah Martin, Greenpeace UK energy campaigner. “It makes no sense for the government to be bending over backwards for the sake of an unproven fossil fuel industry when safe and cheap clean energy is already booming.”