Anti-High Speed 2 (HS2) campaigners plan to take their case to the United Nations and the European Court of Justice after losing an appeal at the Supreme Court.
They also hope to make their objections to the project an election issue.
HS2 Action Alliance said it was “disappointed” the Supreme Court dismissed its case that the UK government had broken European rules in the planning of the HS2 rail link.
Its campaign director Emma Crane told NCE the group still believed it could halt the £42.6bn project.
“We definitely hope so - there is a long way to go [for the scheme],” she said. “This is a deeply flawed project and if you are going to spend £50bn of taxpayers’ money then you have to go through the environmental hoops.”
HS2 Action Alliance believes the government failed to comply with the Strategic Environmental Assessment Directive when seeking planning approval
It says the Supreme Court - which upheld a decision made by the Appeal Court last year - made too narrow an interpretation of the directive when ruling that the government need not comply with it.
The Alliance plans to complain about this interpretation to the European Commission in the hope of being granted the right to take the UK government to the European Court of Justice.
The campaign group also plans to lodge an official complaint with the United Nations, claiming breaches of the Aarhus Convention which links environmental and human rights.
Although the committee has few powers, it is believed a ruling against HS2 could pile pressure on the government.
Another way the Alliance is hoping to increase pressure is through MPs. “We are making MPs aware of the issue. We hope public opinion can make a difference ahead of the election,” said Crane.
Transport minister Baroness Kramer said the government would “press on” with HS2 and that the ruling did not address the issue of whether the line was needed.
“We welcome that the Supreme Court has unanimously rejected the appeal, which addressed technical issues that had no bearing on the need for a new north-south railway,” she said.
“The government’s handling of the project has been fully vindicated by the highest court in the land.”
The Department for Transport said in November that upgrading existing lines to cope with predicted rail demand would cost up to £20bn and require 2,500 weekend line closures, rendering it “untenable”.
It said the economic benefits of faster journeys on a high speed line far outweighed the 9% cost premium, and higher environmental impact, compared with building a standard line.