Campaigners who launched a fresh legal battle to stop what they fear will be serious harm to the Chilterns say they remain "deeply concerned" - after their challenge was thrown out by a judge.

Misbourne Environmental Protection, a group of residents including hydrologists and geologists, fear that the globally rare chalk stream the River Misbourne is at “very significant risk” from pollution because of the tunnels under construction for HS2 in the Chilterns.

MEP applied for a judicial review into the Environment Agency's decision to grant HS2 permission to construct the Chiltern Tunnels after crowdfunding cash from fellow concerned residents - but the review has been refused by Mrs Justice Lieven in what the campaigners said was a "disappointing outcome".

The group raised particular concerns about HS2's use of bentonite – a type of clay that can swell and turn into a gel when dispersed into water – during construction of the tunnels, and the effect it could have on wildlife and the chalk aquifer, which supplies 20 per cent of London’s fresh water.

Liquid bentonite is highly polluting and can damage plants and animals if it gets into watercourses, a safety document produced by Network Rail claims - although HS2 disputes this.

According to a Freedom of Information response from Align JV, HS2’s main works contractor, around 2,500 tonnes of bentonite was “lost” at the Chalfont St Peter vent shaft site up until November 2020.

The judge's decision not to allow the judicial review comes just days after it was announced the eastern leg of HS2 was being scrapped.

MEP said in a statement: "We respectfully thank Justice Lieven for time taken to reach such a difficult and technical decision.

"While MEP understand and accept the judgement, it is clear that protection of the Chilterns aquifer, Misbourne chalk stream and Shardeloes Lake has not been given adequate or necessary protection.

"However, the judge has limited scope within a judicial review process since only errors in law can be considered. The fault originates with the Hybrid Bill process, as does much of the other ongoing damage to the AONB.

"It is yet another example of government's callous disregard for our freshwater and failure to protect our most precious water bodies, such as chalk streams, from damage which is otherwise avoidable.

"We shall be paying close attention to Align JV given their promises to the court, as the entire liability for delivering this tunnel without impacts to the water bodies now rests with them."

Dr Jim Conboy, chairman of MEP, said: “It is ironic that following the cancellation of the eastern leg, and collapse of the business case for HS2, the trail of destruction along the phase one route continues unabated.

"While this campaign did not succeed, it was nevertheless important to make every possible effort to protect the Misbourne and the Chiltern aquifer.”

Chris Wilson, MEP director, added that the decision "shouldn't deflect us from scrutinising this damaging and unnecessary project", saying: "We remain deeply concerned as to the long lasting impacts this project could have on our aquifer and precious chalk stream."

An HS2 spokesman said: “We’re pleased the court has once again confirmed that the Environment Agency went through the right processes when it approved the start of work on the Chiltern Tunnel.

"HS2 Ltd take our responsibilities incredibly seriously and we have a world-leading team of engineers, hydrogeologists, and environmental scientists in place to ensure that the tunnelling operation affords the right protection for the environment, and is completed as safely and efficiently as possible.

"As a result, there has been no significant effect on the aquifer since the start of tunnelling six months ago.”