Prosecutors here are expected to drop all charges against the Rev. Bernard T. Pagano Thursday if the man who has claimed responsibility for the robberies of which the priest is accused enters a guilty plea in open court.

A source close to the prosecution said tonight that Ronald W. Clouser, the 39-year-old U. S. Postal Service employee who dramatically claimed responsibility for the six "Gentleman Bandit" holdups on Monday, has demostrated intimate knowledge of the robberies for which Pagano is on trial.

"Only the perpetrator of the robberies would have been able to detail them to the extent Clouser did," the source said.

As the result of plea bargaining with the Delaware attorney general's office, which has been prosecuting Pagano, Clouser has agreed to plead guilty in three of the six holdups of which the Roman Catholic priest is accused, according to the source.

Clouser would plead guilty to second degree robbery, which carries a prison sentence of two-to-20 years. Pagano is being tried for first degree robbery, which carries three-to -30-year sentences, with a mandatory three years in prison.

Whether Clouser enters the pleas at Pagano's trial Thursday apparently hinges on the outcome of mental tests Clouser is undergoing.If Clouser is found mentally competent to enter the pleas, he is expected to do so, the source said.

If Clouser, who spent six weeks in a mental institution earlier this year is found incompetent to enter a plea, the prosecution plans to request a mistrial. That would give investigators time to get additional psychological evaluations of Clouser, the source said, and police would continue their investigation of the robberies.

Disclosure of the prosecution's strategy tonight was a surprising reversal. Late this afternoon, investigative sources said the effort to obtain Pagano's conviction was expected to resume.

That decision was made, according to sources, after Clouser was placed in a police lineup and four of the holdup victims failed to identify him as the "Gentleman Bandit."

Yesterday, according to police sources, Clouser failed a polygraph test in which he said he was involved in the robberies.

The prosecution source said tonight it had been learned that Clouser had taken a tranquilizer drug before submitting to the polygraph test and it is believed that may have influenced the outcome. The results of such tests are subject to error and are inadmissable in courts.

Finally, the source stressed, it was Clouser's detailed knowledge of the crimes Pagano is accused of that led to the plea bargaining with Clouser.

The prosecutors and police were apparently in dispute on this point, with one police investigator expressing the opinion that Clouser had shown knowledge of nothing that wasn't contained either in newspaper accounts or court files.

The Pagano trial was dramatically halted Monday afternoon when Judge Andrew Christie announced, in front of the jury, that he had received word that another man had "confessed" to the robberies. Several hours later, after Christie sent the jury home, Clouser and his attorney appeared in the courtroom and told the judge that Clouser was "involved" in the robberies.

Clouser, who lives in Brookhaven, Pa., pleaded guilty last May to three armed robberies in Delaware County, Pa. Those robberies were similar in execution to the ones with which Pagano is charged in that the suspect was neatly dressed, wore a dress hat, was polite to his victims and used a small, chrome-plated handgun.

It was because of his mild manners and neat appearance that Delaware police dubbed their suspect the "Gentleman Bandit."

Police in Pennsylvania, when they arrested Clouser last spring, said they found a chrome-plated pistol which they believed was the one used in the Pennsylvania robberies. But the Delaware police have never recovered the pistol, or the approximately $700 involved in their "Gentleman Bandit" case.

If Clouser's psychological condition is found to preclude his pleading guilty, the prosecution is expected to seek a mistrial with the argument that the jury was prejudiced Monday by the judge's announcement that "a Pennsylvania man has confessed to all six offenses" for which Pagano was being tried.

When asked today why he made that announcement in front of the jury, Christie offered this reasoning: If prosecutors decided to go forward with the case, even in light of Clouser's claim, Clouser would probably be called at some point as a witness for the defense. And, the judge said, if prosecutors decided to drop the case altogether because of Clouser's claim, then "it wouldn't make any difference then what the jury heard."

The key to the prosecution's case against Pagano has been the eyewitness identifications of the priest made by seven witnesses to the robberies. Pagano s maintained that he is an innocent victim of mistaken identity. His attorney has stressed throughout the case that only two of the eyewitnesses identified Pagano as the "Gentleman Bandit" before he was arrested and his picture appeared in local papers.

Pagano was charged last February with five counts of armed robbery and one count of attempted armed robbery. At the time he was serving as assistant pastor of St. Mary's Refuge of Sinners Church in Cambridge, Md., but was living part of the time in South St. George's in a small town just outside Wilmington.

Pagano today said a group of his former parishioners from St. Mary's, who have raised over $11,000 for his defense, will come to Wilmington Thursday and will all celebrate mass here.