Protest groups against the HS2 rail project have sent the government a formal letter warning they will take legal action unless it is stopped.
The HS2 Action Alliance said in the letter to Transport Secretary Justine Greening that it would consider seeking a judicial review.
The group said it was concerned about the project's environmental impact.
The £33bn high speed rail line crossing from London and Birmingham and beyond was given the go-ahead last month.
The alliance - a not-for-profit limited company - sent the letter, which accused the government of failing to comply with several legal requirements, on Monday.
It said the Department for Transport failed to comply with "the legally binding requirements of the Strategic Environmental Assessment (SEA) Regulations 2004 and the Conservation of Habitats and Species Regulations 2010."
The SEA regulations included the need for "full strategic environment assessment" and "an assessment of all alternatives to be completed" before the proposals were put forward for public consultation, it claimed.
The challenge is supported by more than 70 local groups and residents' associations, four wildlife trusts and other countryside organisations.
In a statement released on its website, the alliance said it had also sent a letter outlining its concerns regarding the scheme to the European Commission.
Its director, Thomas Crane, said the scheme was an "environmental disaster" and the government had put forward a "shaky business case" for it.
He said: "The Department for Transport and HS2 Ltd have ridden roughshod over public opinion and many expert voices to ignore all viable alternatives in their desperation to promote HS2.
"We are still hopeful that Justine Greening will see sense and halt a project which offers such limited benefit for so much environmental damage.
In January, the Stop HS2 protest group said it had been planning to seek a judicial review and lodge the application within a matter of months. It said it was seeking evidence that the government's decision was flawed.
Mr Crane clarified to the BBC that as the cost of mounting such a challenge was quite expensive, his organisation was issuing the formal notice and was supported by Stop HS2.
An alliance of 18 local authorities working together under the collective name 51m, have also sent a formal notice to Ms Greening asking her to stop the project or face a potential judicial review.
But a Department for Transport spokesman said the project would create jobs and prosperity across the country.
He added: "The line of route between London and the West Midlands has been continually improved to mitigate the impact on those living near it and the environment.
"We believe we have struck the right balance between the reasonable concerns of people living on or near the line - who will be offered a package of compensation measures - the environment and the need to keep Britain moving."
Chris Howe, from pro-HS2 group YestoHS2, said the alternative plans suggested by activists against the scheme would see a cut in local and regional services and fail to provide the capacity needed to cope with predicted future demand.
He said he was disappointed that councils were also considering action and might "spend what could be millions of pounds worth of council taxpayers' cash on fighting a proposal which is of national importance".
Phase one of HS2, between London and Birmingham, is expected to be running by 2026, with an extension to northern England later.