HS2 will divide and destroy hundreds of wildlife sites and nature reserves - including "irreplaceable" ancient woodlands, a report has warned.
The Wildlife Trusts has issued a bleak assessment of the potential environmental impact of the high-speed rail line, which is already facing scrutiny as costs spiral "out of control".
Little more than a week after claims that the project - due to connect London with the North and Midlands - could end up costing upwards of £107bn, The Wildlife Trusts has urged the government to rethink the proposals.
The organisation says it has carried out its "most comprehensive" study to date of how the project could damage the environment and claims that "huge swathes" of natural habitats are at risk.
It could mean extinction for "rarities" such as the dingy skipper butterfly, and danger for barn owls and endangered wildlife such as white-clawed crayfish.
The areas listed as being most at risk are:
Nikki Williams, director of campaigns and policy at The Wildlife Trusts, said the "grim" figures showed why a "greener" approach was needed to HS2 - assuming the project goes ahead.
Doubts were cast over its viability last summer when the Department for Transport announced an independent review to assess its affordability, efficiency and how deliverable it is.
Prime Minister Boris Johnson has been questioned over his support for the rail line, which has become the biggest infrastructure project in Europe.
HS2 Ltd, the company behind the project, says it is committed to delivering a railway that "respects" the environment and will create a "green corridor" along the planned route.
The firm's website claims that 3.4 square miles of new woodlands - made up of seven million trees and shrubs - and another 1.5 square miles of wildlife habitat will be established.
Ms Williams has dismissed those plans as "inadequate" and labelled HS2 Ltd's proposed mitigation for the environmental impact as "amateurish" and in "the wrong place".
She added: "HS2 will destroy precious carbon-capturing habitats if it's allowed to continue in its current form. It will damage the very ecosystems that provide a natural solution to the climate emergency."
Hilary McGrady, director general of National Trust, urged those behind the project to be mindful of their "responsibility to lead by example" and deliver "a net gain for nature".
She said: "We recognise that designing the railway is a long process but plans for HS2 must not end up cutting corners at the expense of the environment."
Extinction Rebellion are among the groups who staunchly oppose HS2, with some protesters having camped out on trees due to be cut down in a bid to delay work.