The UK's only hydraulically fractured shale gas well suffered a previously unreported structural failure, emails obtained by VICE News reveal.

The damage to the Preese Hall site in Lancashire, discovered earlier this year, was contained within the well and there is no evidence that it caused methane or fluids to leak into the atmosphere or surrounding rock formation. However, its public disclosure has prompted calls for the UK to rethink its fledgling fracking industry.

Green Party parliamentarian Caroline Lucas renewed calls for the UK to abandon fracking and told VICE News that the revelations "cast serious doubt" on the government's assurances of its safety.

A series of emails between the Health and Safety Executive (HSE) — the public body responsible for monitoring well integrity — and energy company Cuadrilla Resources from April 2014, released under freedom of information laws, detail what engineers have described to VICE News as a loss of wellbore integrity followed by measures to remediate.

A well integrity failure can be serious, causing gas or fracking fluids to leak. In this case the emails show that there was a leak of gas within the well; however, there was no evidence of any leaks outside the production casing and into the surrounding environment.

Cuadrilla has denied that the emails indicate a loss of wellbore integrity. However, three independent engineers have verified to VICE News that the emails show that well integrity was breached, and a spokesperson for the HSE admitted it could be interpreted as such.

Anthony Ingraffea, Professor of Engineering and Emeritus and Weiss Presidential Teaching Fellow at Cornell University, told VICE News: "It is quite apparent from the language used in [emails between HSE and Cuadrilla] that there was indeed a loss of wellbore integrity followed by attempts to remediate."

He added that "each integrity failure is a special case in terms of environmental impact, because there are so many factors to consider."

John Bissett, the discipline leader in well construction at Robert Gordon University, said: "It is quite clear that well integrity has been breached, technically. How significant that failure is could be a matter for discussion."

He added that the failure is "potentially significant for any planned future wells and fracturing activities."

Mike Hill, an independent chartered engineer based in Lancashire, said: "'Well integrity failure' can mean a leak outside the well or one contained within the well.

"With any well integrity failure there is absolutely a very serious risk of going into the rock formations. It all depends on where it leaks."

The Preese Hall site is no longer operational. A decision had already been taken to abandon the well in December 2013 following two earthquakes in 2011, which scientists believe were caused by fracking at the site.

The earthquakes caused a deformation in the well casing — a less serious incident than a well integrity failure. Following the incident Cuadrilla was reprimanded by ministers for failing to recognise its significance, and for failing to report it to the Department for Energy and Climate Change for six months.

Last year papers released to The Guardian under freedom of information laws showed that Charles Hendry, the then energy minister, said in a letter to the company that the "failure" had exposed "weaknesses in Cuadrilla's performance as a licensee."

No fracking has taken place since the earthquakes; however, a report into the incidents published by the Royal Society and Royal Academy of Engineering in June 2012 concluded that the risk was minimal. The European Union issued an approval for fracking under certain conditions in January 2014.

A fracking boom in the US has driven natural gas production to the highest it has ever been. However, scientists have voiced concerns about the potential health and safety risks posed to people living near fracking sites.

On Friday a government-commissioned report by the UK's chief scientific advisor, Mark Walport, compared the risks of fracking to those posed by thalidomide, tobacco and asbestos.

The report said that fracking arguably offers "a contemporary prospective example" to historical "innovation trajectories that later proved to be problematic."

A spokesperson for the Health and Safety Executive told VICE News: "Some may interpret increased pressure in the annulus as a well integrity issue. The pressure increase in the annulus was well within the design parameters and does not constitute a risk to the health and safety of people; therefore it was not reportable to HSE.

"There was no leak of fluids from the well and the issue has been resolved during the abandonment process. HSE inspectors will continue to monitor the situation."

A spokesperson for Cuadrilla told VICE News: "The well integrity at Preese Hall is secure and always has been since the well was drilled and hydraulically fractured in 2011.

"The Health & Safety Executive is fully appraised of all the operations at Preese Hall and has inspected the site and has confirmed on several occasions that there has been no failure of well integrity at Preese Hall."

A spokesperson for the Environment Agency told VICE News: "There has been no unplanned release of fluids from the Preese Hall well, no evidence to date that the gas has escaped into the environment with the gas isolated within the annulus by the well head structure. Groundwater monitoring is still in place on the site."

Lucas, the Green Party's member of parliament (MP) for Brighton Pavilion, told VICE News: "The government spin machine has been in overdrive trying to convince people to ignore the environmental and health risks of fracking, yet these new revelations cast serious doubt upon their assurances.

"Ministers claim that the serious leaks of gas and fracking fluid that have blighted the industry in the USA couldn't happen here because of strong regulation. But now it seems that, before fracking has even started in a meaningful way, there have been problems with the well casing, and independent experts are saying that well integrity has been breached.