Two environmental protesters campaigning against the HS2 high-speed rail link have walked free from court after a prosecution against them collapsed.

Sarah Green, 63, a Green party member, and Laura Hughes, 37, of Extinction Rebellion, were both charged with aggravated trespass for protesting against work HS2 was carrying out in the Colne Valley nature reserve in Hillingdon, London, home to a variety of fauna and flora including bats, owls and osprey.

Both of the accused repeatedly raised concerns about the destruction of wildlife habitats and damage to 22% of London’s water supply caused by HS2 workers pile-driving into an aquifer on the site. At an earlier hearing, both women pleaded not guilty to charges in relation to obstructing a digger.

The HS2 development has generated huge controversy about cost, usefulness and possible environmental damage. Environmental protesters have established a camp close to the Colne Valley HS2 works site and protesters have staged various demonstrations there in recent months.

The prosecution at Uxbridge magistrates court related to a protest at the Colne Valley site on 11 December 2018. Green was alleged to have climbed on top of an HS2 digger and Hughes was alleged to have locked herself to the digger.

Q&A What is Extinction Rebellion? Show Extinction Rebellion is a protest group that uses non-violent civil disobedience to campaign on environmental issues. Launched in October 2018, with an assembly at Parliament Square to announce a 'declaration of rebellion' against the UK Government, the group has staged regular demonstrations against current environmental policies. More than 1,000 activists were arrested in April 2019 after protesters occupied four sites across London, as well as blocking roads, disrupting a railway line and conducting a protest at Heathrow. Other demonstrations have included a semi-naked protest inside the House of Commons and blockading streets in London, Cardiff, Leeds, Bristol and Glasgow. The group says climate breakdown threatens all life on Earth, and so it is rebelling against politicians who “have failed us”, to provoke radical change that will stave off a climate emergency. The movement has become global with groups set up in countries include the US, Spain, Australia, South Africa and India. Martin Belam Photograph: Anadolu Agency/Anadolu

The district judge presiding over the case, Deborah Wright, dismissed the case after she said it was not possible to prove beyond reasonable doubt that the protesters were trespassing on land in the possession of HS2.

HS2 had previously admitted that a map it provided to the Crown Prosecution Service outlining which part of the site was covered by a high court injunction against protesters was incorrectly labeled.

Two prosecution witnesses who were part of HS2’s security team provided different accounts to the court on Wednesday of exactly where the two protesters were on the site at the time of the alleged offences.

Various maps of land possessed by HS2 and the boundary of the high court injunction were submitted as part of the prosecution case. But there was confusion about exactly what land was and was not owned by HS2.

Wright said: “I would have a reasonable doubt about whether the ladies concerned were trespassing at the time of the alleged offence.”

The court looked at three separate pieces of land. The judge said: “I’m satisfied that two of these pieces of land were in possession of HS2. However, there’s a narrow strip, the third piece of land, about which I have heard no evidence. It is the vicinity of the area where Ms Green and Ms Hughes are said to have trespassed on. This is an area, effectively, of fields. I cannot be sure beyond reasonable doubt that these offences occurred on land that was in the possession of HS2. The defendants are entitled to the benefit of the doubt. I’m going to dismiss the case against them.”

Sailesh Mehta, the counsel for Green, said after the hearing: “These protestors are safeguarding future generations’ right to a valuable resource. An attempt to curtail that has failed today in court. This is a case that should never have been brought before a court.”

Both Green and Hughes said their victory was bittersweet because their concerns about the damage HS2 is causing to the environment remained valid.

Hughes said: “Although I’m happy that this weight has been lifted from me and my children, the weight of climate change and the destruction of my children’s future remains.”

Green said: “The CPS relied on evidence from HS2 which was not accepted by the judge. On one level I feel relieved about today’s outcome, but on another level I’m not at all relieved because our drinking water is still under threat.”

Joe Rukin, of Stop HS2, said: “This is absolutely terrible news for HS2. They have ignored environmental concerns about the project from day one. They can now expect a summer of direct action from protesters concerned about the damage they are causing to the environment.”

An HS2 spokesperson said: “Alongside the economic benefits, HS2 will deliver a new green corridor made up of woodland, wildlife habitats and amenity facilities, designed to blend the line into the landscape and leave a lasting legacy of high-quality green spaces all along the route. At every stage during the construction of the railway, the health and safety of the public and our staff is our priority.”