The Government has been accused in the Supreme Court of "cutting corners" to push through the HS2 national high-speed rail project.
The criticism was made by the HS2 Action Alliance (HS2AA) as it linked with Heathrow Hub and local councils along the proposed route in a bid to force a further assessment of the current scheme to link London, the Midlands and the North.
David Elvin QC, appearing for HS2AA, said the case concerned "the most important strategic rail decision this country has taken at least for a generation". It was was likely to have a huge impact on the environment and many people's lives.
He criticised the Government for failing to consult as widely as it had promised and consider properly alternatives to its preferred scheme.
He asked seven Supreme Court justices, headed by Lord Neuberger, to overturn an appeal court 2-1 majority ruling which went against HS2AA.
He wants the seven to declare that Government transport chiefs unlawfully failed to carry out a strategic environmental assessment (SEA), in breach of a EU Directive.
Questions raised by today's case may have to be referred to the Court of Justice of the European Union (CJEU), delaying the scheme at least for several months, if not longer.
Objectors say it will cost far too much to get HS2, as currently envisaged, up and running. The current estimated cost is £50 billion - but London Mayor Boris Johnson and others have predicted it will eventually cost more than £70 billion.
Mr Elvin told the panel that at least 10 sites of special scientific exercise, more than 50 ancient woodlands, four Wildlife Trust reserves and numerous local wildlife sites were on the proposed HS2 route. Some 170,000 homes lay within a kilometre of it.
In its January 2010 command paper - "High Speed Rail: Investing in Britain's Future - Decisions and Next Steps" (DNS) - the Government set out its strategy, said Mr Elvin.
It was clear the DNS had been followed in the subsequent development of phases one and two of HS2 in the form of a "Y network".
The DNS would also exert considerable influence when the Government requested development consent for HS2 from Parliament in a Hybrid Bill, said Mr Elvin.
Hybrid Bills are used to reflect both the public importance of proposed legislation and also its impact on private individuals or groups.
Phase One of the Y network involves creating a high-speed link between London and Birmingham, allowing through trains to run on to the West Coast main line to service cities further north.
Proposals include a new interchange station at Old Oak Common in west London, with a connection to Crossrail, the Heathrow Express, Great Western main line and local public transport, and a direct link to the Channel Tunnel.
Phase Two proposes to extend the project from the West Midlands further north to Manchester on the western fork and to Leeds on the eastern fork.
Mr Elvin said of the DNS: "There was no assessment of the Y network as a whole - the assessment of phase two has not happened yet."
Referring to the limitations of the consultation exercise, he said: "What we have here is a cutting of corners."
He added: "This is not simply an academic exercise seeking to identify breaches of European law. It is a real defect because the Government intended the consultation exercise to be wide ranging."
The seven judges - Lord Neuberger, Lady Hale, Lord Kerr, Lord Reed, Lord Mance, Lord Sumption and Lord Carnwath - are also considering 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).
The councils say the parliamentary hybrid Bill procedure is not capable of achieving the objectives of the EIAD. They say it does not meet the standard of scrutiny required by European law because the HS2 proposals have all-party support and there will be a whipped vote of MPs.
A Department for Transport spokesman said: "HS2 is absolutely vital for this country if we are to meet the urgent capacity needs we face.
"Attempts to obstruct HS2 have already been firmly rejected by two courts.
"The Government will continue to defend any challenge in the Supreme Court, but strongly believes Parliament is the right place to debate the merits of HS2, not the courts."
Hilary Wharf, director, HS2AA, said: "It's is a sad day when hard working tax-payers have to take the Government to the highest court in the land to ensure that it protects irreplaceable environments for future generations.
"The very fact that we have been given the right to appeal to the Supreme Court and seven judges are sitting shows the vital importance of this case, not just for the right decisions to be made around HS2, but for future national infrastructure projects."
A thousand people donated £50 each over five days to fund the Supreme Court action.
Yesterday the man due to spearhead the project told MPs HS2 had never just been about speed.
Sir David Higgins, who is soon to take over as chairman of HS2 Ltd, said it was also about improving capacity and about connecting major cities.
Currently chief executive of Network Rail (NR), Sir David told the House of Commons Transport Committee that he had been keen to get involved with HS2 because he feared the project was becoming "a political football" and it was "too important" to be so regarded.
The Supreme Court hearing is expected to last two days, when the judges will reserve their decision.