Federal authorities are investigating charges by a government informer that he paid bribes to Rep. Nicholas Mavroules (D-Mass.) and to a longtime aide to House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.).

Peter Robert Aver, 35, a former bar and restaurant owner, testified in U.S. District Court here today that he paid Mavroules, then mayor of Peabody, to solve his inspection and liquor license problems.

He also testified that James Rowan, an adminstrative assistant to O'Neill who has served in his Boston office for about 25 years, took payoffs to reduce a debt on a Small Business Administration loan.

Edward Harrington, U.S. attorney for Boston, issued a curt statement: "I confirm that federal authorities are in receipt of information regarding those allegations, and they are currently under investigation."

A spokesman for O'Neill, Gary Hymel, said "The speaker says he doesn't know anything about this."

Rowan responded: "I can truthfully say I don't know the guy [Aver], I never met him, I never spoke to him or had any dealing of any kind with him; he's just trying to bail himself out by naming everybody in the city."

Mavroules, a freshman congressman who won the seat vacated by Rep. Michael Harrington, today issued a denial of the allegations and said he met Aver only once, in the early 1970s at the Coach and Six, a restaurant in Peabody owned by Aver at the time. Mavroules said his wife and children were with him at that meeting.

"I have never met him, seen him nor spoken to him since; nor have I at any time had any association or dealings with him," the statement said. "I unequivocally and without reservation deny any allegation he has made or might make."

Mavroules also warned: ". . . whoever might read or report anything he might say in the future should consider the posture and position from which he speaks and his background."

Aver has a lengthy criminal record and he had conceded under oath that he participated in a wide range of illegal activities including arson for hire and bribery, for which he was not convicted.

He has been the principal government witness in the recent trial of two men charged with the murder of five persons at the Black Friars Club in Boston and the bribery trial of two state Alcoholic Beverages Commission investigators.

He is also cooperating with federal investigations involving state tax department workers and banking officials. He admitted Tuesday paying a public official $5,000 for a liquor license.

Aver's allegations surfaced Tuesday at an unrelated theft trial when he revealed he had been granted immunity from state and federal prosecution in return for his testimony about political corruption. He had been charged with receiving stolen goods from the two men on trial here.

He made the statements during cross-examination be defense attorney Paul Caradona, who was trying to discredit Aver's credibility as a witness by pointing out the government deal with the informer.

William McDermott, director of the FBI's political corruption unit in Boston, said the testimony came as a shock to the bureau. "We weren't expecting any discussion on this. I'm not sure what it's going to do to our investigation."

Aver's testimony, he said, is a key part of some of those investigations.

Aver changed his name from Auer after serving a jail sentence for larceny. He received a new identity, under the federal witness protection program, and was moved with his wife and a young daughter to another state because he said he feared for his life.

Aver, a native of Cambridge, Mass., who grew up in East Boston, testified Tuesday that he has received $7,300 in aid from the federal government since last October along with moving expenses and promises of a job.