The Stop HS2 campaign has approached Public Interest Lawyers and is launching a funding appeal for a judicial review because it believes the evidence is "stacking up" against the rail link between London and Birmingham.
The group believes that the project has been progressed further than it should prior to public consultation.
In April, the law firm wrote a pre-action protocol letter to the transport secretary expressing concern that European procurement requirements had not been met.
The lawyers claim that Arup was appointed to plan HS2 without tendering by an exchange of letters.
At a summit organised by Buckinghamshire County Council and district councils this month, Buckinghamshire cabinet member for planning and environment Martin Tett said: "The environmental case doesn't stack up. The government wants to quantify the national interest but ignore the costs.
"We owe it to future generations to protect the Buckinghamshire countryside and the Chilterns area of outstanding natural beauty."
Transport secretary Philip Hammond is due to announce the preferred route later this autumn. Consultation is scheduled to start early in 2011, with a government decision later in the year.
After detailed design and an environmental impact assessment, the scheme will be presented to parliament as a hybrid bill. Work is scheduled to start in 2015.