WASHINGTON -- Supreme Court Justice Harry A. Blackmun, 85, has decided to retire and plans to announce it later today.

The senior justice, best known as the author of the Roe vs. Wade abortion decision in 1973, told the White House several days ago of his plan.

Ending a quarter-century on the court, Justice Blackmun has become its most liberal member. He and Justice John Paul Stevens are the dwindling remnant of the court's once-dominant liberal bloc.

He was considered a moderate when appointed by President Richard M. Nixon in 1970. But Justice Blackmun became closely allied with two liberal justices, William J. Brennan Jr., now retired, and the late Thurgood Marshall.

The most recent gesture demonstrating how liberal Justice Blackmun had become came just weeks ago: He announced that he would no longer support any execution because he had become convinced that America's scheme of capital punishment longer could function constitutionally.

He has often referred to himself as "Old No. 3," a reference to the fact that Mr. Nixon made him a justice after the first two choices had been rejected by the Senate.

A gentle man, he has seldom spoken on the bench except to display a keen worry over the plight of deprived or injured individuals whose cases came before the court.

Some observers have criticized Justice Blackmun for his very personal display of emotions as a jurist, suggesting that he has seen himself as the court's moral conscience.

He will be known primarily as the judge who found a constitutional right to privacy allowing women to end pregnancies by abortion with the consent of their doctor.