The supreme court has rejected the final outstanding appeals by objectors to the HS2 high-speed rail link, who had claimed the decision-making process was unlawful.
The appeals, brought by three campaign groups, came on two grounds after judges last year struck down seven areas of challenge to the government's handling of the £42.6bn scheme.
Judges unanimously dismissed claims that the government was failing to comply with the strategic environmental assessment directive, and that the hybrid bill before parliament would breach the environmental impact assessment.
The appeal was brought by the HS2 Action Alliance (HS2AA), local authorities and Heathrow Hub Ltd, a private grouping hoping to build an HS2 station near the airport.
Responding to the ruling, the transport minister Lady Kramer 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. The government's handling of the project has been fully vindicated by the highest court in the land.
"We will now continue to press ahead with the delivery of HS2. The new north-south line will provide extra space for more trains and more passengers to travel on the network, delivering additional capacity where it is most needed. HS2 will also generate thousands of jobs across the UK and provide opportunities to boost skills."
The supreme court's verdict marks the end of the judicial reviews into HS2, two years after opponents first launched legal action. Nine out of 10 areas of challenge were dismissed by the high court last March, although the campaigners' one victory did force the government into rerunning a consultation on compensation arrangements for property owners affected by building HS2.
Of these cases, seven were taken to appeal and again went in favour of the government in July 2013. Only two of these were taken to the supreme court, which further reaffirmed the legality of the government's decision-making on HS2.
The court refused applications for matters to be referred to the European court.
However, campaigners said they would continue to pursue avenues in Europe. HS2AA said it considered that the supreme court's refusal to refer the matter to the European court breached its obligations under the EU treaty. It said it would bring complaints to Europe's compliance committee and the European commission.
Hilary Wharf of HS2AA said: "We will continue to press the government to meet its environmental obligations. The government should be safeguarding our environment for future generations and the simple fact is HS2 is an unnecessary and hugely damaging project environmentally."
A Department for Transport spokesman responded: "It can be an expensive mistake to resort to legal challenge. We want to work with people and have a constructive dialogue, and we hope that people will recognise this and respond. We hope that the judgment in this case will reassure everyone with an interest in HS2 that the department is acting in a lawful manner, thereby avoiding the need for further costly litigation."
While the legal road now appears clear for the government, at least for the first phase between London and Birmingham of the high-speed network, one of its key environmental supporters has indicated it will withdraw its backing. The Campaign for the Protection of Rural England, which has to date given conditional support, said it was "increasingly hard to see HS2 as any sort of green project".
Its chief executive, Shaun Spiers, said the group had reconsidered as the government was set to expand airports and launch a major roadbuilding programme. Writing on his blog, he said: "It seems we are to have HS2 and new roads and runways. That was never part of the deal. It is not clear what this does for the business case for HS2 – there are only so many passengers – but it blows a hole in the environmental case."
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