Gov. Robert F. McDonnell’s administration has started examining how the nation’s largest known uranium deposit could be mined.

But some critics complain that the study, which is expected to be completed by the end of the year, is being conducted behind closed doors with little input from the public.

Virginia Uranium, a company that wants to mine the site, had lobbied aggressively to lift a three-­decade ban on uranium mining this year, flying legislators to France and Canada to visit mines and donating to their campaigns. Instead, McDonnell (R) recommended in January that the state needed to further study the impact of excavating the site before the General Assembly considers lifting the moratorium.

Uranium was found three decades ago in Coles Hill, near Chatham, a small town in Pittsylvania County, near the southern corner of the state. The deposit, located under farmland, begins near the surface and runs about 1,500 feet deep.

Virginia Uranium said tests indicate there is about 119 million pounds of uranium — worth as much as $10 billion — below the surface. It is the world’s seventh-largest known deposit — enough to supply all U.S. nuclear power plants for about two years or satisfy Virginia’s demands for 75 years.

McDonnell created a multi-agency group — composed of staff from the state’s Department of Health, Department of Environmental Quality and Department of Mines, Minerals and Energy — charged with studying the site and drafting regulations.

To help with the study, the state hired Wright Environmental Management at a cost of about $500,000, according to Matt Conrad, the governor’s deputy chief of staff. The state has had discussions with a second consultant but has not finalized a contract.

Groups concerned about health and safety are questioning why the state is looking into how regulations might be written and hiring costly consultants when legislators have yet to act. They also say a uranium mine could contaminate natural resources, cause illness, and have long-term effects on plants and animals.

“Virginia Uranium did not have votes in the General Assembly this year, so in our opinion the governor came to the rescue,” said Glen Besa, director of the Sierra Club’s Virginia chapter. “The outcome is already known.”

In a letter to the administration this year, the Piedmont Environmental Council, the Virginia League of Conservation Voters and the Sierra Club criticized the process.

“This action . . . has the unfortunate effect of taking the process away from the commonwealth’s elected representatives and placing it behind closed doors,” they wrote. “In particular, we are troubled by the possibility of little public involvement in this critical process.”

McDonnell spokeswoman Taylor Thornley said that state officials are merely studying the issue and that no regulations would be adopted while the moratorium is in place.

“What we are providing the General Assembly is information they would want to consider if they choose to lift the moratorium, such as whether there are gaps that need filling in existing programs or agencies so as to ensure that uranium can be mined and milled safely in Virginia,” she said.

The state’s working group will accept public comments during four open meetings — in June, August, October and November — and on a Web site.

After criticism, McDonnell’s administration held a meeting with environmental groups to provide “complete and accurate information” on uranium mining regulations. The media were invited to observe but were told by McDonnell’s office that there was no time for interviews.

McDonnell’s chief of staff, Martin Kent, said in a letter to legislators that people had misunderstood the process.

“We have developed a detailed work plan that ensures that important level of openness and transparency,” he wrote.

If the General Assembly lifts the ban, it will take a year for the state to adopt regulations before companies can apply to mine.

Virginia Uranium officials declined to comment last week on the process. They have said in the past that additional safeguards have been put in place since mining at Coles Hill was first considered. They also said any mining would be under federal regulations.

In December, a long-awaited study by the National Academy of Sciences said “steep hurdles” had to be surmounted before Virginia lifted its ban. The 302-page report did not address the particular 200-acre site and whether it is suitable for mining.

The report said uranium could be mined but that Virginia Uranium would have to take measures to protect workers, the public and the environment in Virginia, which has no experience unearthing a radioactive element.

The National Academy of Sciences is holding a series of meetings to brief the public on its two-year study. Meetings were held in Fairfax County, Charlottesville and Richmond. Its final meeting will be Thursday in Virginia Beach.