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."
Anti-link group Stop HS2 expressed disappointment with today's judgment and expressed doubts that MPs will be able to vet the project properly during its progress through Parliament in light of Government readiness to "rush" it through.
It warns that there are dangers involved for future generations if the project is allowed to "blast through".
Group chair Penny Gaines said: "We are disappointed with this decision. It's clear from the judgment that the judges are expecting Members of Parliament to look carefully at the HS2 Hybrid Bill, and not simply follow the party whip.
"But the Government has shown that it is eager to rush this legislation through, in the hope that no one spots the problems with it.
"However, even though they have been taken to court over these matters, HS2 Ltd are still treating the environmental regulations as box-ticking operations.
"The ongoing environmental consultation has been twice extended by the House of Commons Standing Orders Committee and the House of Lords Standing Orders Committee, due to failings in the process by HS2 Ltd.
"We hope that other MPs look closely at the environmental issues surrounding the Bill, and do not simply wave it through.
"This is not a game: if it goes ahead, HS2 will affect the areas it blasts through for ever, both in urban areas and the countryside. We have a duty to future generations to make sure that the environment is protected."
Stop HS2 campaign manager Joe Rukin attacked HS2 Ltd, the company responsible for developing and promoting the project, saying that, although the company had been found not to have behaved illegally, it had acted " unfairly and irresponsibly right from the start" and rushed proposals through " without due diligence".
Hillingdon Council said it was proposing to take the case to Europe, saying: " We are now actively exploring with our legal team the possibility of making a complaint to the European Commission.
"We do not believe that this decision today endorses a fair and open process for decision-making."
The council has also launched a further judicial review challenge with the HS2 Action Alliance, arguing that the Government failed to carry out a strategic environmental assessment before issuing safeguarding directions.
Council leader Ray Puddifoot said: ''Although we understand the reasons why the Supreme Court has dismissed the appeals, we will continue in our fight against HS2 and will do everything possible to protect our residents against this flawed project."