Investigation work ahead of a proposed £2.4bn road tunnel near Stonehenge should have been done before a public consultation, campaigners have said.
Ground investigation works by Highways England have begun at the Countess Roundabout in Amesbury.
The Stonehenge Alliance, which argues a tunnel would damage the prehistoric landscape, also said data from previous investigations had been withheld.
Highways England said all "relevant" information had already been submitted.
The government is set to make a ruling on the plans by 13 November.
Dr Kate Fielden, from the Stonehenge Alliance, said it was "unlikely" results of the latest work to test ground conditions would be known before the ruling is due.
"Will the data from these latest investigations... be available to the secretary of state, and not only available, but fully interpreted?"
Dr Fielden added that results of other geology and ground conditions investigations elsewhere in the landscape had still not been made public.
"We've asked for hydro-geological and geological data from work that's been done in the landscape... in 2019, and we just haven't had it.
"Data has been withheld from people who have a legitimate interest in bringing their evidence and opinions to the public examination of the scheme.
"We think that this information is crucial to knowing whether or not the scheme is feasible."
Highways England senior project manager David Bullock said: "Our ongoing survey work in no way pre-empts the outcome of the Development Consent Order (DCO) Examination as all the relevant and widely available analysis and information has already been submitted as part of the planning process.
"The current ground investigations are taking place to help inform bidders with their tenders, which will enable contractors to improve their design proposals and put us in a position to be able to complete the procurement process and ready ourselves for construction, providing consent is given."
Funding for the two-mile (3.2km) dual-carriageway tunnel was announced in Chancellor Rishi Sunak's first budget in March.
More than 120,000 people have signed a petition started by the Stonehenge Alliance.
A public consultation into the scheme ended in April 2018.
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