A Prince George's County judge yesterday denied Robert Ashley Smith's request for a new trial, dealing a severe blow to the 24-year-old's hopes of proving that he had not set a 1972 fire that killed a couple who lived across the street from Smith's Bowie home.

The decision by Circuit Court Judge Vincent J. Femia, following a three-day hearing, ended one of the final chapters in the Smith family's lengthy legal battle to see their son set free. After reading his ruling, Femia ordered Smith sent back to the Maryland State penitentiary, where the young man is serving a life term.

After the decision was issued yesterday before a packed courtroom, Smith's father angrily told reporters, "We are going to appeal it as long as it takes . . ." a loud reiteration of the pledge he made after his son's murder conviction in 1973.

Robert E. Smith held his wife's arm and added: "Eventually we will run out of law and get justice."

The 61-year-old paint salesman said the evidence in the three-day hearing "clearly showed my son was innocent."

Judge Femia, who pointed out at the beginning of the trial that the post-conviction hearing was not to determine guilt or innocence, concentrated yesterday on the technical legal questions of Smith's case, not on the facts of the of the crime.

Judge Femia's ruling yesterday concentrated on the central issue of Smith's appeal - whether police officers had formally arrested Smith shortly before he made a statement to authorities allegedly implicating himself in the crime.

According to court records, Smith was taken to county police headquarters the night after the fire and questioned intensively before making the statement that, Smith's parents say, was the key factor in his conviction.

If Smith actually had been arrested before he made the statement his attorney contended, he would have been entitled to a new trial, since the arrest would have been illegal.

Judge Fema rejected that contention, ruling that there had been no arrest until Smith gave the statement. Femia based this decision on passages of Smith's own 1973 trial testimony, when Smith admitted he did not believe he had been arrested.

Smith's failure to get a new trial yesterday comes after almost five years of appeals. His case has even been appealed to the Supreme Court, which refused to hear it.

The only hope the legal system now offers Smith and his family is the chance that the Maryland Court of Special Appeals will again review the case, or that a federal court will consider it, his attorneys say.

Smith's attorney's say they will file an appeal within the next 30 days.

Smith was convicted in the fire Nov. 29, 1972, at 12305 Starlight La. that killed George and Lucille Brown.

Prince George's police asked Smith, who then was 19 and described by some neighbors as arrogant and strange, to come to their Bureau of Investigation to make a statement about the fire.

After 18 hours of questioning, police said, Smith gave a statement.

According to the court record, Smith admitted throwing gasoline under the front door of the Browns' home and igniting it.

Smith's attorneys argued there was no arson at the Brown home, but that faulty electrial wiring and a burning cigarette inside the home caused the fire.

At the three-day hearing, Smith's attorneys argued there was still another possible explanation for scorching found on the home's front door. They produced an expert witness who testified that polyurethane foam inside the sofa of thehome could have been ignited by a cigarette and produced a flammable gas that exploded, scorching the door.

Smith's attorney argued that since this evidence came to light after Smith's trial, he should be given a new opportunity to present the information in his defense.

Judge Femia ruled that no new evidence could be used as Smith's defense since his trial had been completed.

The judge did rule that Smith's arson conviction should be overturned because of a recent U.S. Supreme Court division on doublejeopardy.