A special investigative bureau will soon be formed to probe allegations of government corruption in Virginia, a state government source said today.

The new unit will be established administratively by Gov. John N. Dalton and will report directly to Secretary of Public Safety H. Selwyn Smith, the source said. It will not operate under the direction of the state police, criticized for its performance in an expanding probe of alleged state purchasing corruption.

Creation of the new bureau will be proposed by the state crime commission. Its chairman, Sen. Stanley C. Walker (D-Norfolk), was quoted by Associated Press today as saying the advisory panel is formulating a plan to reorganize state investigations of governmental corruption.

Walker said no final plan has been adopted by the commission, but the source said he is confident that Dalton and Smith will establish the separate investigate bureau administratively without seeking new legislation.

Smith refused in an interview to discuss reorganization plans. "This is the wrong time to talk about that kind of thing," he said. "Everyone is taking pot shots at the state police, but they are doing an outstanding job on this investigation and it is no time to say anything that would affect their morale."

Dalton, however, said in a recent news conference that he is "concerned" that it took newspaper stories in the Norfolk Ledger-Star to revive an investigation of alleged kickbacks to former state printing officials in the division of purchases and supply.

State police investigated allegations of corruption in printing purchases in 1976, but Richmond Commonwealth's Attorney Aubrey Davis declined to seek indictments on the basis of their probe.

A grand jury was called this year and has been helped in its investigation by witnesses who agreed to testify after being quoted anonymously in the Ledger-Star stories by reporter David Chandler.

Reports of plans to create a special investigative bureau to probe government corruption came amid official acknowledgements that the state police investigation of alleged purchasing corruption is expanding.

Smith said in an interview that he has "heard James River" food products "and printing contracts with the welfare department discussed" in connection with the investigation, but refused to elaborate.

"James River" is a brand name for food sold by Smithfield Ham and Products Co. of Smithfield, Va. Alton Gwaltney, Smithfield vice president, told United Press International that he is unaware of the investigation.

The proposal to create a special investigative unit to probe government corruption is a response to complaints that the state police, traditionally devoted to traffic law enforcement, lack the expertise to cope with white-collar crime.

Attorney General Marshall Coleman said in an interview today that he favors setting up a new investigative division to deal with white-collar crime in and out of government, but believes it should be assigned to his office.

The attorney general now has only limited law enforcement powers. Almost all crimes investigated by state and local police are prosecuted by city and county commonwealth's attorneys.

"I think it is logical to place investigations of governmental wrongdoing and crimes that involve more than one local jurisdiction under the attorney general," Coleman said.

He also called for legislation that would permit state investigators to obtain subpoenas to examine some financial records without calling a grand jury and creation of statewide grand juries to deal with crimes that cross city and county lines.