The Government has vowed to "press ahead" with the HS2 national high-speed rail link after winning a key legal victory in the Supreme Court.
The highest court in the land unanimously dismissed accusations by objectors that the Government was "cutting corners" to push the estimated £50 billion project through Parliament, in breach of European environmental laws.
But the objectors are now planning to take their case to Europe, threatening more delays.
They include a number of local authorities along the route and action groups and residents' associations opposed to the link, plus Heathrow Hub, which is promoting integration of the project with Heathrow Airport.
They challenged the Transport Secretary's decision to introduce in Parliament hybrid Bills to authorise the development between London, Birmingham and the North.
It was argued that the hybrid Bill procedure was inappropriate and would not allow proper scrutiny of the proposals in regard to environmental concerns.
Government transport chiefs were also accused of failing to carry out a strategic environmental assessment (SEA), in breach of an EU directive.
But seven Supreme Court judges - Lord Neuberger sitting with Lady Hale, Lord Kerr, Lord Reed, Lord Mance, Lord Sumption and Lord Carnwath - all rejected the challenge.
A further challenge brought by local councils led by the London Borough of Hillingdon over a second EU directive - the Environmental Impact Assessment Directive (EIAD) - was also dismissed.
The councils had argued that the hybrid Bill procedure - used to reflect both the public importance of proposed legislation and also its impact on private individuals or groups - did not comply with the requirements of the EIAD.
Dismissing the challenges and giving the hybrid Bill route the all-clear, the judges ruled there was "no reason to suppose that MPs will be unable properly to examine and debate the proposed project".
They also ruled that there was no need for the court to refer the case to the Court of Justice of the European Union (CJEU).
Their decision was welcomed by Transport Minister Baroness Kramer, who said: " 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.
"We will now continue to press ahead with the delivery of HS2."
Leading campaign group HS2 Action Alliance warned that - although the legal battle can be taken no further in the UK - it would be continued in Europe.
Alliance director Hilary Wharf said: "We always knew this would be a long fight.
"We will continue to press the Government to meet its environmental obligations. The Government should be safeguarding our environment for future generations and it is simply the fact that HS2 is an unnecessary, hugely damaging project environmentally."
The Alliance's campaign director, Emma Crane, said: "We will make a complaint to the European Commission to say the UK Government has not complied with its European law obligations on the environment. We are speaking to our lawyers."