The Virginia attorney general's office, in an apparently unprecedented action, has asked an Alexandria judge to vacate the death sentence of Wilbert Lee Evans for the 1981 murder of a deputy sheriff because of sentencing errors that the American Civil Liberties Union branded "outrageous."

In a letter made public yesterday, Assistant Attorney General Jerry P. Slonaker acknowledged that the Alexandria jury that recommended Evans be executed was given records of Evans' criminal past that were "seriously misleading and/or . . . defective." Despite those errors Alexandria prosecutors are free under a recently enacted law to again seek Evans' execution in the state's electric chair.

Chan Kendrick, Virginia director of the ACLU, which sponsored Evans' challenge to his sentence, yesterday called the state's handling of the case "outrageous. It illustrates the imperfect system we've got and shows why people shouldn't be executed," Kendrick said.

J. Lloyd Snook III, a Charlottesville lawyer representing five death row inmates, said the attorney general's letter marks the first time the state has "confessed error" in a capital case since Virginia's current death penalty statute was enacted in 1977.

Slonaker, in a letter to Circuit Court Judge Wiley R. Wright Jr., said that, among other errors, the jury was told of four past convictions in North Carolina in which Evans had no lawyer. Such convictions have been inadmissible to a sentencing jury since 1980.

Slonaker also said Virginia authorities recently confirmed that North Carolina prosecutors had dropped a charge against Evans -- assaulting a police officer with a knife -- that was presented to the Alexandria jury as a conviction. Still another North Carolina charge against Evans was portrayed erroneously to the jury here as two separate convictions, Slonaker said.

The accuracy of Evans' record was critical to the jurors, whose task under Virginia law was to determine whether Evans' "future dangerousness" justified the death sentence.

In his letter to the judge, Slonaker came close to conceding that Evans' death sentence was likely to be reversed on appeal by the federal courts. " . . .In the interest of justice, the [state] is constrained to concede that Wilbert Evans' current death sentence cannot be sustained," he wrote.

The attorney general's action leaves the next move up to Wright, who imposed the death sentence. If he vacates the sentence, then Commonwealth's Attorney John Kloch, who prosecuted Evans, will have to decide whether to ask that a second jury be impaneled to consider whether Evans should die or get life in prison.

If Wright does not vacate the sentence, then Evans' lawyers must press their appeal in the federal courts. Neither the prosecutor nor judge indicated yesterday what he would do.

Evans was sentenced by Wright in June 1981 for the fatal shooting of Alexandria Deputy William Truesdale during an escape attempt at the Alexandria city jail. Evans was captured within minutes in a parking lot in the city's Old Town section.

Under emergency legislation that took effect March 28, Evans' case may be taken to a new jury for possible resentencing to execution. Gov. Charles S. Robb sought the change after another death row defendant, James T. Clark Jr., convicted of murder-for-hire in Fairfax County, won commutation of his death penalty based on sentencing errors.

A spokesman for the Virginia attorney general's office said yesterday that state officials had not delayed admitting error in the Evans case until the new law had taken effect.

Asked whether he knew Evans had been convicted repeatedly in North Carolina without benefit of a lawyer, prosecutor Kloch said: "I assumed he had one."

North Carolina officials reviewed the records after Alexandria lawyer Jonathan Shapiro, acting at the request of the ACLU, filed a petition last April to block Evans' execution less than a week before he was to die. An affidavit submitted earlier this month by a Raleigh court clerk confirmed the errors.

In his letter, Slonaker said the mistakes occurred "unbeknownst to the prosecution or defense counsel."

Shapiro's petition alleged, however, that "The Commonwealth knew or should have known" about the flaws and called Kloch's use of much of the information about Evans' record "remote, erroneous, improperly prejudicial and duplicative."

The petition also sharply criticized Evans' court-appointed trial lawyers, Alexandria attorneys E. Blair Brown and Stefan C. Long, for numerous alleged omissions, including "the complete and utter failure to present any evidence at the sentencing phase despite the availability of such evidence."

"There's a big difference between allegations and a final determination," Brown said yesterday. "If that the petition is what it took to get him off death row, sure, I'm upset, but I don't know that I'm that upset."