WASHINGTON (AP) _ Federal officials on Monday rebutted allegations that the Soviets used a stand-in for would-be defector Miroslav Medvid to tell U.S. officials he had decided he wanted to return to the Soviet Union.

Medvid, a seaman from a Soviet grain freighter, had twice jumped ship into the Mississippi River in the late evening and early morning of Oct. 24-25, only to be put back on ship by agents of the U.S. Border Patrol.

Depite the denials, by spokesmen for the State Department and the Immigration and Naturalization Service, a senator and a congressman renewed calls for investigations into the incident.

State Department spokesman Charles Redman said that officials who saw the sailor aboard the ship on Oct. 25, and questioned him on Oct. 28 and 29, positively identified him as Medvid.

''Allegations that the Soviets pulled a switch and a substitute was interviewed are without foundation,'' Redman told reporters at a briefing.

''The articles reporting that have dealt with some sensational allegations and some unproved theories,'' the spokesman said. ''They have ignored or misrepresented key facts which make it completely clear that no substitutions occurred.''

''An INS representative who originally interviewed seaman Medvid onshore on Oct. 24 confirmed that the individual he saw in the sick bay of the Marshal Koniev (the Soviet freighter) on Oct. 25 was the same person he had interviewed on the shore the day before,'' Redman said.

INS spokesman Duke Austin acknowledged that the Border Patrol agents who sent Medvid back to the ship on the night of Oct. 24 had not followed established guidelines, and have been demoted and disciplined. But he said that one of them positively identified Medvid on Oct. 25.

The denials of a swtich did little to quiet congressional calls for a renewed investigation of the Medvid incident, which occurred three weeks before the Geneva meeting in November between President Reagan and Kremlin leader Mikhail Gorbachev.

Sen. Gordon Humphrey, R.-N.H., in a speech on the Senate floor, said ''it is clear ... that the State Department did not take all necessary steps to ascertain Medvid's true wishes before allowing him to be returned to his ship'' on Oct. 29.

''An independent investigation is needed to determine whether there was any unethical or unlawful conduct on the part of any U.S. employee or official ...'' said Humphrey. ''The American people need to know who was involved with this case at all levels ... ''

Humphrey said 61 senators have cosponsored a resolution calling for creation of a special panel to probe the incident.

One congressional source, speaking on condition that he not be identified, said there was suspicion that administration officials may have agreed to allow Medvid to sail away on the Soviet freighter to avoid a superpower dispute before the summit meeting in Geneva.

Rep. Edward F. Feighan, D-Ohio, said in Cleveland that he would urge a House probe of the affair to assure ''that American agencies learn their lesson and ensure in the future that anyone who attempts to defect to the United States is given much more opportunity than Mirsolav Medvid.''

Austin said that an internal INS study of the case, conducted last December, resulted in a decision to provide greater emphasis on guidelines requiring agents to consult with their superiors in asylum cases.

Those who believe that the Soviets may have pulled a switch in the Medvid case have cited apparent discrepancies in descriptions of the sailor and photographs taken at different intervals during the incident.

An official form completed by the border patrol, and based on measurements, described Medvid as 5-feet-10 and 174 pounds.

A Navy doctor who examined a sailor in the freighter's sick bay 24 hours later described him as of ''short stature, approximately 150'' pounds.

Redman, however, said that a more thorough subsequent examination by the Navy doctor set Medvid's height at 5-10 and his weight at 170.

Advocates of the switch theory also say that photographs taken by Navy Lt. James Geltz of the sailor interviewed on Oct. 28-29 show he was not the same man photographed by INS agents on Oct. 24.

FBI specialists have examined both sets of pictures, however, and have found them inconclusive, said James Lucier, an aide to Sen. Jesse Helms, R.- N.C., who has backed the call for a Senate probe.