Opponents of the Government's HS2 high-speed rail scheme asked the High Court today to declare the £34 billion project legally flawed and send it back for reconsideration.
The first phase of HS2 would see a new, high-speed railway line running through Tory heartlands from London to Birmingham.
The scheme has polarised opinion, with many residents' groups and some councils bitterly opposing it, but supporters point out the benefits a reduction in journey times between the UK's two biggest cities could bring.
The legal challenge at London's High Court, which is expected to last seven days, could potentially delay the scheme for years if successful.
The case is being fought in several parts. Today, a QC for 51m, an alliance of local authorities opposing HS2, accused the Government of being guilty of fundamental failures.
These include failing to undertake a strategic environmental assessment, or to arrange a consultation process "that allows informed responses on the true merits and demerits of the proposals".
Nathalie Lieven QC, appearing for 51m, said there had been no proper environmental assessment of the impact of the "entire project", or the full cumulative impacts of the first phase.
She told Mr Justice Ouseley: "It is a decision involving the purchase and demolition of hundreds of homes. In Camden (London) alone between 200-500 dwellings are affected.
"They have no idea what the figure will be north of Birmingham."
Ms Liven said the scheme must also be blocked because of a failure to comply with the EU's Environmental Impact Assessment Directive.
The second application for judicial review is by campaign group HS2 Action Alliance (HS2AA) and concerns the legality of compensation arrangements for homeowners living near the proposed HS2 line.
Lawyers for HS2AA say homeowners are in danger of not being properly compensated following a "fundamentally flawed" consultation process.
They say some 172,000 properties within 1km of the first phase are being affected by "a generalised blight" since the proposals were announced.
The 51m group's chairman, Martin Tett, who is also leader of Buckinghamshire County Council, said of the decision to resort to legal action: "We are doing this with immense reluctance. However, we feel that we have been left with no alternative.
"This immensely expensive project ignores the Davies Review of aviation policy, has an abysmal business case and represents extremely poor value for hard-pressed taxpayers' money. Far better alternatives exist to build the infrastructure the country desperately needs more quickly and at far lower cost.
"We cannot let the Department for Transport, with its atrocious record on managing key investment decisions, not answer for why these have been ignored."
A Department for Transport spokesman said: "HS2 will bring cities closer together, drive regeneration, tackle overcrowding and stimulate economic growth.
"While it would not be appropriate to comment on the specific claims, the Government is confident that the decisions on HS2 have been taken lawfully and fairly and it is vigorously defending these legal challenges."
The High Court is also hearing challenges from the Heathrow Hub group and from Aylesbury Golf Club in Buckinghamshire.
HS2AA director Hilary Wharf said: "The Government is trying to force through HS2 without following proper process. We have two strong cases coming to court that expose how the Government has been unlawfully taking shortcuts in its decision-making processes. Our cases concern environmental issues and the consultation on compensation.
"We are confident in our arguments and while Government has consistently ignored the compelling case against HS2, it can't ignore the courts."
The Transport Secretary decided in January this year to proceed with the construction of HS2. The proposals are for a high-speed rail link between London, Birmingham, Leeds and Manchester - the "Y" network - with trains running initially at speeds of up to 225mph, and later potentially reaching 250mph.
The network will also provide links to Heathrow Airport and the Channel Tunnel via the existing High Speed 1 (HS1) line.
The proposals are for the Y network to be built in two phases. The first phase will run between London and the West Midlands.
The second phase is planned to extend HS2 beyond the West Midlands to Manchester and Leeds and include direct high-speed services to a new Heathrow Airport station.
The court was told it will also add in further stations in Manchester, the East Midlands and South Yorkshire, Leeds and Heathrow.
But neither the route for the second phase, nor the location of second phase stations, have been identified in Government documents, the judge was told.
Phase one, expected to cost £16 billion, would alone have very significant impact on the environment and homeowners, with a large number of properties facing demolition, with at least 216 in Camden to accommodate the expansion of Euston Station, Ms Lieven told the judge.
Even though it was decided in January to proceed with the entire project, it was impossible to assess the impact on the environment of the second phase, even in broad terms, because no route had been published.
This was in breach of environmental impact regulations requiring assessments to be carried out, argued Ms Lieven.
With regard to phase one, route alterations had been made that had not been subject to consultation, even though in a number of instances "these alterations create greater impacts on individual properties and the environment than was previously the case".
Ms Lieven listed other legal flaws which, she argued, meant the whole project must be reconsidered.